diff --git a/Program.cs b/Program.cs
index a39a728..7812652 100644
--- a/Program.cs
+++ b/Program.cs
@@ -14,11 +14,30 @@ class Program {
         var input = File.ReadAllText(args[0]);
         var parser = new StParser(input);
         var prog = parser.ParseProgram();
-        var emitter = new BytecodeEmitter();
-        emitter.Compile(prog);
-        File.WriteAllBytes(args[1], emitter.BuildBinary());
-        Console.WriteLine($"Wrote {args[1]}: consts={emitter.ConstantsCount}, vars={emitter.VarCount}, code={emitter.CodeLength}");
-        return 0;
+        
+        if (parser.HasErrors) {
+            Console.WriteLine($"Compilation failed with {parser.Errors.Count} errors:");
+            foreach (var error in parser.Errors) {
+                Console.WriteLine($"Error at line {error.Line}: {error.Message}");
+            }
+            return 1;
+        }
+
+        if (prog == null) {
+            Console.WriteLine("Compilation failed: invalid program structure");
+            return 1;
+        }
+
+        try {
+            var emitter = new BytecodeEmitter();
+            emitter.Compile(prog);
+            File.WriteAllBytes(args[1], emitter.BuildBinary());
+            Console.WriteLine($"Wrote {args[1]}: consts={emitter.ConstantsCount}, vars={emitter.VarCount}, code={emitter.CodeLength}");
+            return 0;
+        } catch (Exception ex) {
+            Console.WriteLine($"Internal compiler error: {ex.Message}");
+            return 1;
+        }
     }
 }
 
@@ -26,13 +45,33 @@ class Program {
 public enum VarType { Int8=1, Int16=2, Int32=3, Byte=4, Bool=5 }
 public abstract class StNode{}
 public class ProgramNode:StNode{ public List<VarDecl> Vars=new(); public List<Stmt> Stmts=new(); }
-public class VarDecl:StNode{ public string Name; public VarType Type; public Expr? Init; }
+public class VarDecl:StNode{ 
+    required public string Name; 
+    public VarType Type; 
+    public Expr? Init; 
+}
 
 public abstract class Stmt:StNode{}
-public class AssignStmt:Stmt{ public string Target; public Expr Expr; }
-public class IfStmt:Stmt{ public Expr Cond; public List<Stmt> ThenStmts=new(); public List<Stmt> ElseStmts=new(); }
-public class WhileStmt:Stmt{ public Expr Cond; public List<Stmt> Body=new(); }
-public class ForStmt:Stmt{ public string Var; public Expr Start, End, Step = new IntExpr(1); public List<Stmt> Body=new(); }
+public class AssignStmt:Stmt{ 
+    required public string Target; 
+    required public Expr Expr; 
+}
+public class IfStmt:Stmt{ 
+    required public Expr Cond; 
+    public List<Stmt> ThenStmts=new(); 
+    public List<Stmt> ElseStmts=new(); 
+}
+public class WhileStmt:Stmt{ 
+    required public Expr Cond; 
+    public List<Stmt> Body=new(); 
+}
+public class ForStmt:Stmt{ 
+    required public string Var; 
+    required public Expr Start; 
+    required public Expr End; 
+    public Expr Step = new IntExpr(1); 
+    public List<Stmt> Body=new(); 
+}
 
 public abstract class Expr:StNode{}
 public class IntExpr:Expr{ public int Value; public IntExpr(int v){Value=v;} }
@@ -50,204 +89,402 @@ public enum TokType {
     PROGRAM, VAR, END_VAR, END_PROGRAM,
     EOF
 }
-public class Token{ public TokType Type; public string Text; public Token(TokType t,string s){Type=t;Text=s;} }
+public class Token{ 
+    public TokType Type; 
+    public string Text; 
+    public int Line;
+    public Token(TokType t, string s, int line) { Type=t; Text=s; Line=line; }
+}
+
+public class CompileError {
+    public int Line;
+    public string Message;
+    public CompileError(int line, string msg) { Line=line; Message=msg; }
+}
 
 public class StLexer {
-    private readonly string src; private int i;
+    private readonly string src;
+    private int i;
+    private int currentLine = 1;
+    public List<CompileError> Errors = new();
     public StLexer(string s){src=s;}
     char Peek()=> i<src.Length?src[i]:'\0';
-    char Next()=> i<src.Length?src[i++]:'\0';
+    char Next(){ 
+        if (i >= src.Length) return '\0';
+        char c = src[i++];
+        if (c == '\n') currentLine++;
+        return c;
+    }
+
+    void AddError(string msg) => Errors.Add(new CompileError(currentLine, msg));
 
     public Token NextToken() {
         while (char.IsWhiteSpace(Peek())) Next();
-        if (Peek()=='\0') return new Token(TokType.EOF,"");
+        if (Peek()=='\0') return new Token(TokType.EOF,"", currentLine);
 
         if (char.IsLetter(Peek())||Peek()=='_'){
             var sb=new StringBuilder();
+            int startLine = currentLine;
             while (char.IsLetterOrDigit(Peek())||Peek()=='_') sb.Append(Next());
             var s=sb.ToString().ToUpperInvariant();
             return s switch {
-                "PROGRAM"=>new Token(TokType.PROGRAM,s),
-                "VAR"=>new Token(TokType.VAR,s),
-                "END_VAR"=>new Token(TokType.END_VAR,s),
-                "END_PROGRAM"=>new Token(TokType.END_PROGRAM,s),
-                "IF"=>new Token(TokType.IF,s),
-                "THEN"=>new Token(TokType.THEN,s),
-                "ELSE"=>new Token(TokType.ELSE,s),
-                "END_IF"=>new Token(TokType.END_IF,s),
-                "WHILE"=>new Token(TokType.WHILE,s),
-                "DO"=>new Token(TokType.DO,s),
-                "END_WHILE"=>new Token(TokType.END_WHILE,s),
-                "FOR"=>new Token(TokType.FOR,s),
-                "TO"=>new Token(TokType.TO,s),
-                "BY"=>new Token(TokType.BY,s),
-                "END_FOR"=>new Token(TokType.END_FOR,s),
-                _=>new Token(TokType.IDENT,s)
+                "PROGRAM"=>new Token(TokType.PROGRAM,s,startLine),
+                "VAR"=>new Token(TokType.VAR,s,startLine),
+                "END_VAR"=>new Token(TokType.END_VAR,s,startLine),
+                "END_PROGRAM"=>new Token(TokType.END_PROGRAM,s,startLine),
+                "IF"=>new Token(TokType.IF,s,startLine),
+                "THEN"=>new Token(TokType.THEN,s,startLine),
+                "ELSE"=>new Token(TokType.ELSE,s,startLine),
+                "END_IF"=>new Token(TokType.END_IF,s,startLine),
+                "WHILE"=>new Token(TokType.WHILE,s,startLine),
+                "DO"=>new Token(TokType.DO,s,startLine),
+                "END_WHILE"=>new Token(TokType.END_WHILE,s,startLine),
+                "FOR"=>new Token(TokType.FOR,s,startLine),
+                "TO"=>new Token(TokType.TO,s,startLine),
+                "BY"=>new Token(TokType.BY,s,startLine),
+                "END_FOR"=>new Token(TokType.END_FOR,s,startLine),
+                _=>new Token(TokType.IDENT,s,startLine)
             };
         }
 
         if (char.IsDigit(Peek())) {
             var sb=new StringBuilder();
+            int startLine = currentLine;
             while(char.IsDigit(Peek())) sb.Append(Next());
-            return new Token(TokType.INT,sb.ToString());
+            return new Token(TokType.INT,sb.ToString(),startLine);
         }
 
-        if (Peek()==':'){ Next(); if(Peek()=='='){Next(); return new Token(TokType.ASSIGN,":=");} }
+        int tokenLine = currentLine;
+        if (Peek()==':'){ 
+            Next(); 
+            if(Peek()=='='){
+                Next(); 
+                return new Token(TokType.ASSIGN,":=",tokenLine);
+            } 
+            AddError("Expected '=' after ':' for assignment");
+            // Bei einem einzelnen ':' geben wir EOF zurück und stoppen das Parsen
+            i--; // Gehen einen Schritt zurück, damit der fehlerhafte ':' Token beim nächsten Mal neu gelesen wird
+            return new Token(TokType.EOF,"",tokenLine);
+        }
         if (Peek()=='<'){
             Next();
-            if (Peek()=='='){Next(); return new Token(TokType.LE,"<=");}
-            if (Peek()=='>'){Next(); return new Token(TokType.NEQ,"<>");}
-            return new Token(TokType.LT,"<");
+            if (Peek()=='='){Next(); return new Token(TokType.LE,"<=",tokenLine);}
+            if (Peek()=='>'){Next(); return new Token(TokType.NEQ,"<>",tokenLine);}
+            return new Token(TokType.LT,"<",tokenLine);
         }
         if (Peek()=='>'){
             Next();
-            if (Peek()=='='){Next(); return new Token(TokType.GE,">=");}
-            return new Token(TokType.GT,">");
+            if (Peek()=='='){Next(); return new Token(TokType.GE,">=",tokenLine);}
+            return new Token(TokType.GT,">",tokenLine);
         }
-        if (Peek()=='='){Next();return new Token(TokType.EQ,"=");}
+        if (Peek()=='='){Next();return new Token(TokType.EQ,"=",tokenLine);}
 
         char c=Next();
-        return c switch {
-            ';'=>new Token(TokType.SEMI,";"),
-            '('=>new Token(TokType.LPAREN,"("),
-            ')'=>new Token(TokType.RPAREN,")"),
-            '+'=>new Token(TokType.PLUS,"+"),
-            '-'=>new Token(TokType.MINUS,"-"),
-            '*'=>new Token(TokType.MUL,"*"),
-            '/'=>new Token(TokType.DIV,"/"),
-            _=>throw new Exception($"Unexpected '{c}'")
-        };
+        if (c == ';') return new Token(TokType.SEMI,";",tokenLine);
+        if (c == '(') return new Token(TokType.LPAREN,"(",tokenLine);
+        if (c == ')') return new Token(TokType.RPAREN,")",tokenLine);
+        if (c == '+') return new Token(TokType.PLUS,"+",tokenLine);
+        if (c == '-') return new Token(TokType.MINUS,"-",tokenLine);
+        if (c == '*') return new Token(TokType.MUL,"*",tokenLine);
+        if (c == '/') return new Token(TokType.DIV,"/",tokenLine);
+        
+        AddError($"Unexpected character '{c}'");
+        return new Token(TokType.EOF,"",tokenLine); // Skip invalid character
     }
 }
 
 // === PARSER ===
 public class StParser {
-    StLexer lex; Token cur;
-    public StParser(string s){lex=new StLexer(s);cur=lex.NextToken();}
-    void Next()=>cur=lex.NextToken();
-    void Expect(TokType t){if(cur.Type!=t)throw new Exception($"Expected {t}, got {cur.Type}");Next();}
+    StLexer lex; 
+    Token cur;
+    public List<CompileError> Errors => lex.Errors;
+    public bool HasErrors => Errors.Count > 0;
 
-    public ProgramNode ParseProgram(){
+    public StParser(string s){
+        lex=new StLexer(s);
+        cur=lex.NextToken();
+    }
+
+    void Next()=>cur=lex.NextToken();
+
+    void AddError(string msg) => lex.Errors.Add(new CompileError(cur.Line, msg));
+
+    bool Expect(TokType t){
+        if(cur.Type!=t) {
+            AddError($"Expected {t}, got {cur.Type}");
+            return false;
+        }
+        Next();
+        return true;
+    }
+
+    public ProgramNode? ParseProgram(){
         var p=new ProgramNode();
-        Expect(TokType.PROGRAM);
+        if (!Expect(TokType.PROGRAM)) return null;
+        
         if(cur.Type==TokType.IDENT) Next();
+        
         if(cur.Type==TokType.VAR){
             Next();
-            while(cur.Type==TokType.IDENT) p.Vars.Add(ParseVarDecl());
-            Expect(TokType.END_VAR);
+            while(cur.Type==TokType.IDENT) {
+                var varDecl = ParseVarDecl();
+                if (varDecl == null) return null;
+                p.Vars.Add(varDecl);
+            }
+            if (!Expect(TokType.END_VAR)) return null;
         }
-        while(cur.Type!=TokType.END_PROGRAM&&cur.Type!=TokType.EOF)
-            p.Stmts.Add(ParseStmt());
-        Expect(TokType.END_PROGRAM);
+        
+        while(cur.Type!=TokType.END_PROGRAM && cur.Type!=TokType.EOF) {
+            var stmt = ParseStmt();
+            if (stmt == null) return null;
+            p.Stmts.Add(stmt);
+        }
+        
+        if (!Expect(TokType.END_PROGRAM)) return null;
         return p;
     }
 
-    VarDecl ParseVarDecl(){
-        string name=cur.Text; Expect(TokType.IDENT);
-        string tname=cur.Text.ToLowerInvariant(); Expect(TokType.IDENT);
-        VarType vt=tname switch{
-            "int8"=>VarType.Int8,"int16"=>VarType.Int16,"int32"=>VarType.Int32,"byte"=>VarType.Byte,"bool"=>VarType.Bool,
-            _=>throw new Exception($"Unknown type {tname}")
-        };
-        Expr? init=null;
-        if(cur.Type==TokType.ASSIGN){Next();init=ParseExpr();}
-        Expect(TokType.SEMI);
-        return new VarDecl{Name=name,Type=vt,Init=init};
-    }
-
-    Stmt ParseStmt(){
-        if(cur.Type==TokType.IF) return ParseIf();
-        if(cur.Type==TokType.WHILE) return ParseWhile();
-        if(cur.Type==TokType.FOR) return ParseFor();
-        if(cur.Type==TokType.IDENT){
-            var n=cur.Text; Next(); Expect(TokType.ASSIGN);
-            var e=ParseExpr(); Expect(TokType.SEMI);
-            return new AssignStmt{Target=n,Expr=e};
+    VarDecl? ParseVarDecl(){
+        if (cur.Type != TokType.IDENT) {
+            AddError("Expected identifier for variable declaration");
+            return null;
         }
-        throw new Exception($"Unexpected token {cur.Type} in stmt");
+        string name=cur.Text; 
+        Next();
+
+        if (cur.Type != TokType.IDENT) {
+            AddError("Expected type name");
+            return null;
+        }
+        string tname=cur.Text.ToLowerInvariant(); 
+        Next();
+
+        VarType? vt = tname switch {
+            "int8" => VarType.Int8,
+            "int16" => VarType.Int16,
+            "int32" => VarType.Int32,
+            "byte" => VarType.Byte,
+            "bool" => VarType.Bool,
+            _ => null
+        };
+        
+        if (vt == null) {
+            AddError($"Unknown type '{tname}'");
+            return null;
+        }
+
+        Expr? init=null;
+        if(cur.Type==TokType.ASSIGN){
+            Next(); // consume :=
+            init=ParseExpr();
+            if (init == null) {
+                AddError($"Expected expression after ':=' in variable declaration");
+                return null;
+            }
+        }
+        
+        if (!Expect(TokType.SEMI)) return null;
+        
+        return new VarDecl{Name=name,Type=vt.Value,Init=init};
     }
 
-    IfStmt ParseIf(){
-        Next();
-        var cond=ParseExpr(); Expect(TokType.THEN);
+    Stmt? ParseAssign() {
+        // Der Aufrufer hat bereits geprüft, dass wir bei einem IDENT sind
+        string target = cur.Text;
+        Next(); // consume identifier
+        if (cur.Type != TokType.ASSIGN) {
+            AddError($"Expected ':=' after identifier '{target}'");
+            return null;
+        }
+        Next(); // consume :=
+        var e = ParseExpr();
+        if (e == null) return null;
+        if (!Expect(TokType.SEMI)) return null;
+        return new AssignStmt{Target=target, Expr=e};
+    }
+
+    Stmt? ParseStmt(){
+        switch(cur.Type) {
+            case TokType.IF: return ParseIf();
+            case TokType.WHILE: return ParseWhile();
+            case TokType.FOR: return ParseFor();
+            case TokType.IDENT:
+                return ParseAssign();
+            default:
+                AddError($"Unexpected token {cur.Type} in statement");
+                return null;
+        }
+    }
+
+    IfStmt? ParseIf(){
+        Next(); // IF
+        var cond=ParseExpr(); 
+        if (cond == null) return null;
+        if (!Expect(TokType.THEN)) return null;
+
         var node=new IfStmt{Cond=cond};
-        while(cur.Type!=TokType.ELSE&&cur.Type!=TokType.END_IF)
-            node.ThenStmts.Add(ParseStmt());
+        while(cur.Type!=TokType.ELSE && cur.Type!=TokType.END_IF && cur.Type!=TokType.EOF) {
+            var stmt = ParseStmt();
+            if (stmt == null) return null;
+            node.ThenStmts.Add(stmt);
+        }
+
         if(cur.Type==TokType.ELSE){
             Next();
-            while(cur.Type!=TokType.END_IF)
-                node.ElseStmts.Add(ParseStmt());
+            while(cur.Type!=TokType.END_IF && cur.Type!=TokType.EOF) {
+                var stmt = ParseStmt();
+                if (stmt == null) return null;
+                node.ElseStmts.Add(stmt);
+            }
         }
-        Expect(TokType.END_IF); Expect(TokType.SEMI);
+
+        if (!Expect(TokType.END_IF)) return null;
+        if (!Expect(TokType.SEMI)) return null;
         return node;
     }
 
-    WhileStmt ParseWhile(){
+    WhileStmt? ParseWhile(){
         Next(); // WHILE
-        var cond=ParseExpr(); Expect(TokType.DO);
+        var cond=ParseExpr(); 
+        if (cond == null) return null;
+        if (!Expect(TokType.DO)) return null;
+
         var ws=new WhileStmt{Cond=cond};
-        while(cur.Type!=TokType.END_WHILE)
-            ws.Body.Add(ParseStmt());
-        Expect(TokType.END_WHILE); Expect(TokType.SEMI);
+        while(cur.Type!=TokType.END_WHILE && cur.Type!=TokType.EOF) {
+            var stmt = ParseStmt();
+            if (stmt == null) return null;
+            ws.Body.Add(stmt);
+        }
+
+        if (!Expect(TokType.END_WHILE)) return null;
+        if (!Expect(TokType.SEMI)) return null;
         return ws;
     }
 
-    ForStmt ParseFor(){
+    ForStmt? ParseFor(){
         Next(); // FOR
-        string varName = cur.Text; Expect(TokType.IDENT);
-        Expect(TokType.ASSIGN);
-        Expr start = ParseExpr();
-        Expect(TokType.TO);
-        Expr end = ParseExpr();
+        if (cur.Type != TokType.IDENT) {
+            AddError("Expected identifier for FOR loop variable");
+            return null;
+        }
+        string varName = cur.Text;
+        Next(); // consume identifier
+        
+        if (cur.Type != TokType.ASSIGN) {
+            AddError($"Expected ':=' after identifier '{varName}'");
+            return null;
+        }
+        Next(); // consume :=
+        var start = ParseExpr();
+        if (start == null) return null;
+
+        if (!Expect(TokType.TO)) return null;
+        var end = ParseExpr();
+        if (end == null) return null;
+
         Expr step = new IntExpr(1);
         if(cur.Type==TokType.BY){
             Next();
-            step = ParseExpr();
+            step = ParseExpr() ?? step;
         }
-        Expect(TokType.DO);
+
+        if (!Expect(TokType.DO)) return null;
+
         var fs = new ForStmt{Var=varName, Start=start, End=end, Step=step};
-        while(cur.Type!=TokType.END_FOR)
-            fs.Body.Add(ParseStmt());
-        Expect(TokType.END_FOR); Expect(TokType.SEMI);
+        while(cur.Type!=TokType.END_FOR && cur.Type!=TokType.EOF) {
+            var stmt = ParseStmt();
+            if (stmt == null) return null;
+            fs.Body.Add(stmt);
+        }
+
+        if (!Expect(TokType.END_FOR)) return null;
+        if (!Expect(TokType.SEMI)) return null;
         return fs;
     }
 
-    Expr ParseExpr()=>ParseCompare();
-    Expr ParseCompare(){
-        var l=ParseAddSub();
+    Expr? ParseExpr() => ParseCompare();
+    
+    Expr? ParseCompare(){
+        var l = ParseAddSub();
+        if (l == null) return null;
+        
         while(cur.Type is TokType.LT or TokType.GT or TokType.LE or TokType.GE or TokType.EQ or TokType.NEQ){
-            var op=cur.Type; Next();
+            var op=cur.Type; 
+            Next();
             var r=ParseAddSub();
+            if (r == null) return null;
             l=new BinaryExpr(l,op,r);
         }
         return l;
     }
-    Expr ParseAddSub(){
+    
+    Expr? ParseAddSub(){
         var l=ParseMulDiv();
+        if (l == null) return null;
+        
         while(cur.Type==TokType.PLUS||cur.Type==TokType.MINUS){
-            var op=cur.Type;Next();var r=ParseMulDiv();l=new BinaryExpr(l,op,r);
+            var op=cur.Type;
+            Next();
+            var r=ParseMulDiv();
+            if (r == null) return null;
+            l=new BinaryExpr(l,op,r);
         }
         return l;
     }
-    Expr ParseMulDiv(){
+    
+    Expr? ParseMulDiv(){
         var l=ParsePrimary();
+        if (l == null) return null;
+        
         while(cur.Type==TokType.MUL||cur.Type==TokType.DIV){
-            var op=cur.Type;Next();var r=ParsePrimary();l=new BinaryExpr(l,op,r);
+            var op=cur.Type;
+            Next();
+            var r=ParsePrimary();
+            if (r == null) return null;
+            l=new BinaryExpr(l,op,r);
         }
         return l;
     }
-    Expr ParsePrimary(){
-        if(cur.Type==TokType.INT){int v=int.Parse(cur.Text);Next();return new IntExpr(v);}
-        if(cur.Type==TokType.IDENT){string n=cur.Text;Next();return new VarExpr(n);}
-        if(cur.Type==TokType.LPAREN){Next();var e=ParseExpr();Expect(TokType.RPAREN);return e;}
-        throw new Exception($"Unexpected token {cur.Type}");
+    
+    Expr? ParsePrimary(){
+        int startLine = cur.Line;
+        switch(cur.Type) {
+            case TokType.INT:
+                if (!int.TryParse(cur.Text, out var v)) {
+                    AddError($"Invalid integer literal '{cur.Text}'");
+                    return null;
+                }
+                Next();
+                return new IntExpr(v);
+                
+            case TokType.IDENT:
+                string n=cur.Text;
+                Next();
+                return new VarExpr(n);
+                
+            case TokType.LPAREN:
+                Next();
+                var e=ParseExpr();
+                if (e == null) return null;
+                if (!Expect(TokType.RPAREN)) return null;
+                return e;
+                
+            default:
+                AddError($"Unexpected token {cur.Type} in expression");
+                return null;
+        }
     }
-}
+    }
+
 
 // === BYTECODE ===
 public class BytecodeEmitter {
     List<int> consts=new(); Dictionary<string,Symbol> syms=new(); List<byte> code=new();
-    class Symbol{public string Name;public VarType Type;public int Index;}
+    class Symbol {
+        required public string Name;
+        public VarType Type;
+        public int Index;
+    }
     public int ConstantsCount=>consts.Count; public int VarCount=>syms.Count; public int CodeLength=>code.Count;
 
     public void Compile(ProgramNode p){
@@ -264,11 +501,21 @@ public class BytecodeEmitter {
         EmitByte(0xF0); // Program End
     }
 
+    Dictionary<TokType, byte> opCodes = new() {
+        {TokType.PLUS, 0x10}, {TokType.MINUS, 0x11}, {TokType.MUL, 0x12}, {TokType.DIV, 0x13},
+        {TokType.LT, 0x14}, {TokType.GT, 0x15}, {TokType.LE, 0x16}, {TokType.GE, 0x17},
+        {TokType.EQ, 0x18}, {TokType.NEQ, 0x19}
+    };
+
     void EmitStmt(Stmt s){
         switch(s){
             case AssignStmt a:
+                Symbol? symbol = null;
+                if (!syms.TryGetValue(a.Target, out symbol)) {
+                    throw new Exception($"Undeclared variable '{a.Target}'");
+                }
                 EmitExpr(a.Expr);
-                EmitByte(0x03); EmitU16((ushort)syms[a.Target].Index);
+                EmitByte(0x03); EmitU16((ushort)symbol.Index);
                 break;
 
             case IfStmt iff:
@@ -293,9 +540,13 @@ public class BytecodeEmitter {
                 break;
 
             case ForStmt f:
+                Symbol? forSymbol = null;
+                if (!syms.TryGetValue(f.Var, out forSymbol)) {
+                    throw new Exception($"Undeclared variable '{f.Var}'");
+                }
                 // Initialisierung: var := start
                 EmitExpr(f.Start);
-                EmitByte(0x03); EmitU16((ushort)syms[f.Var].Index);
+                EmitByte(0x03); EmitU16((ushort)forSymbol.Index);
 
                 int cmpPos = code.Count;  // Position des Vergleichs
                 EmitExpr(new VarExpr(f.Var));
@@ -322,8 +573,6 @@ public class BytecodeEmitter {
                 EmitExpr(f.End);
                 EmitByte(0x03); EmitU16((ushort)syms[f.Var].Index);
                 break;
-
-            default: throw new Exception($"Unknown stmt {s.GetType().Name}");
         }
     }
 
@@ -333,16 +582,18 @@ public class BytecodeEmitter {
             case IntExpr ie:
                 int ci=AddConst(ie.Value);EmitByte(0x01);EmitU16((ushort)ci);break;
             case VarExpr ve:
-                int vi=syms[ve.Name].Index;EmitByte(0x02);EmitU16((ushort)vi);break;
+                Symbol? symbol = null;
+                if (!syms.TryGetValue(ve.Name, out symbol)) {
+                    throw new Exception($"Undeclared variable '{ve.Name}'");
+                }
+                EmitByte(0x02);EmitU16((ushort)symbol.Index);break;
             case BinaryExpr be:
+                if (!opCodes.ContainsKey(be.Op)) {
+                    throw new Exception($"Unknown operator '{be.Op}'");
+                }
                 EmitExpr(be.L);EmitExpr(be.R);
-                EmitByte(be.Op switch{
-                    TokType.PLUS=>0x10,TokType.MINUS=>0x11,TokType.MUL=>0x12,TokType.DIV=>0x13,
-                    TokType.LT=>0x14,TokType.GT=>0x15,TokType.LE=>0x16,TokType.GE=>0x17,
-                    TokType.EQ=>0x18,TokType.NEQ=>0x19,
-                    _=>throw new Exception("bad op")
-                }); break;
-            default: throw new Exception("bad expr");
+                EmitByte(opCodes[be.Op]);
+                break;
         }
     }
 
diff --git a/bin/Debug/net8.0/Compiler.dll b/bin/Debug/net8.0/Compiler.dll
index 1561c98..7258a5f 100644
Binary files a/bin/Debug/net8.0/Compiler.dll and b/bin/Debug/net8.0/Compiler.dll differ
diff --git a/bin/Debug/net8.0/Compiler.pdb b/bin/Debug/net8.0/Compiler.pdb
index 313a9e8..66e8de9 100644
Binary files a/bin/Debug/net8.0/Compiler.pdb and b/bin/Debug/net8.0/Compiler.pdb differ
diff --git a/bin/Debug/net8.0/input.st b/bin/Debug/net8.0/input.st
deleted file mode 100644
index d0f483a..0000000
--- a/bin/Debug/net8.0/input.st
+++ /dev/null
@@ -1,7 +0,0 @@
-PROGRAM MyProg
-VAR
-a;
-x;
-END_VAR
-x := a + 5;
-END_PROGRAM
\ No newline at end of file
diff --git a/obj/Debug/net8.0/Compiler.AssemblyInfo.cs b/obj/Debug/net8.0/Compiler.AssemblyInfo.cs
index 837a151..2c1028c 100644
--- a/obj/Debug/net8.0/Compiler.AssemblyInfo.cs
+++ b/obj/Debug/net8.0/Compiler.AssemblyInfo.cs
@@ -13,7 +13,7 @@ using System.Reflection;
 [assembly: System.Reflection.AssemblyCompanyAttribute("Compiler")]
 [assembly: System.Reflection.AssemblyConfigurationAttribute("Debug")]
 [assembly: System.Reflection.AssemblyFileVersionAttribute("1.0.0.0")]
-[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+f6aef1f0d176c5c9d337f8595797f1eb1e58a56b")]
+[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+dbd7715193158898901ea48bb685edce0d11c047")]
 [assembly: System.Reflection.AssemblyProductAttribute("Compiler")]
 [assembly: System.Reflection.AssemblyTitleAttribute("Compiler")]
 [assembly: System.Reflection.AssemblyVersionAttribute("1.0.0.0")]
diff --git a/obj/Debug/net8.0/Compiler.AssemblyInfoInputs.cache b/obj/Debug/net8.0/Compiler.AssemblyInfoInputs.cache
index a361074..26bdf22 100644
--- a/obj/Debug/net8.0/Compiler.AssemblyInfoInputs.cache
+++ b/obj/Debug/net8.0/Compiler.AssemblyInfoInputs.cache
@@ -1 +1 @@
-b797b871506999fe8d363a1f65cdb2c12ea0f8c34caff9806f36efedbe800822
+48b1033f57085a9800e40e95f5bd07aadedd1a0f80d0f7a99d777225eeff6382
diff --git a/obj/Debug/net8.0/Compiler.dll b/obj/Debug/net8.0/Compiler.dll
index 1561c98..7258a5f 100644
Binary files a/obj/Debug/net8.0/Compiler.dll and b/obj/Debug/net8.0/Compiler.dll differ
diff --git a/obj/Debug/net8.0/Compiler.pdb b/obj/Debug/net8.0/Compiler.pdb
index 313a9e8..66e8de9 100644
Binary files a/obj/Debug/net8.0/Compiler.pdb and b/obj/Debug/net8.0/Compiler.pdb differ
diff --git a/obj/Debug/net8.0/ref/Compiler.dll b/obj/Debug/net8.0/ref/Compiler.dll
index bd097f2..e528250 100644
Binary files a/obj/Debug/net8.0/ref/Compiler.dll and b/obj/Debug/net8.0/ref/Compiler.dll differ
diff --git a/obj/Debug/net8.0/refint/Compiler.dll b/obj/Debug/net8.0/refint/Compiler.dll
index bd097f2..e528250 100644
Binary files a/obj/Debug/net8.0/refint/Compiler.dll and b/obj/Debug/net8.0/refint/Compiler.dll differ