An internal compiler error occurs in the "while h != null:" line. Somehow everything works fine using "while h is not null". import System class Helper: [getter(Stringlist)] stringlist as (string) h = Helper() while h != null: print text for text in h.Stringlist //h = nextHelper() [...] Entering Boo.Lang.Compiler.Steps.EmitAssembly ASSEMBLY LOADED: <dynamic>Program, Version=0.0.0.0 ASSEMBLY LOADED: c:\windows\assembly\gac\isymwrapper\1.0.5000.0__b03f5f7f11d50a3a\isymwrapper.dl l Program.boo(10,7): Internal compiler error on node 'h': System.ArgumentNullException: Value cann ot be null. Parameter name: local at System.Reflection.Emit.ILGenerator.Emit(OpCode opcode, LocalBuilder local) at Boo.Lang.Compiler.Steps.EmitAssembly.OnReferenceExpression(ReferenceExpression node) at Boo.Lang.Compiler.Ast.ReferenceExpression.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) Leaving Boo.Lang.Compiler.Steps.EmitAssembly Program.boo(10,7): BCE0055: Boo.Lang.Compiler.CompilerError: Internal compiler error: Value cannot b e null. Parameter name: local. ---> System.ArgumentNullException: Value cannot be null. Parameter name: local at System.Reflection.Emit.ILGenerator.Emit(OpCode opcode, LocalBuilder local) at Boo.Lang.Compiler.Steps.EmitAssembly.OnReferenceExpression(ReferenceExpression node) at Boo.Lang.Compiler.Ast.ReferenceExpression.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) — End of inner exception stack trace — at Boo.Lang.Compiler.Ast.DepthFirstVisitor.OnError(Node node, Exception error) at Boo.Lang.Compiler.Steps.AbstractVisitorCompilerStep.OnError(Node node, Exception error) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) at Boo.Lang.Compiler.Steps.EmitAssembly.EmitRawBranchTrue(Expression expression, Label label) at Boo.Lang.Compiler.Steps.EmitAssembly.EmitBranchTrue(BinaryExpression expression, Label label) at Boo.Lang.Compiler.Steps.EmitAssembly.EmitBranchTrue(Expression expression, Label label) at Boo.Lang.Compiler.Steps.EmitAssembly.OnWhileStatement(WhileStatement node) at Boo.Lang.Compiler.Ast.WhileStatement.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node[] array) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(NodeCollection collection) at Boo.Lang.Compiler.Steps.EmitAssembly.OnBlock(Block block) at Boo.Lang.Compiler.Ast.Block.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) at Boo.Lang.Compiler.Steps.EmitAssembly.OnMethod(Method method) at Boo.Lang.Compiler.Ast.Method.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node[] array) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(NodeCollection collection) at Boo.Lang.Compiler.Steps.EmitAssembly.EmitTypeDefinition(TypeDefinition node) at Boo.Lang.Compiler.Steps.EmitAssembly.OnClassDefinition(ClassDefinition node) at Boo.Lang.Compiler.Ast.ClassDefinition.Accept(IAstVisitor visitor) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node node) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(Node[] array) at Boo.Lang.Compiler.Ast.DepthFirstVisitor.Visit(NodeCollection collection) at Boo.Lang.Compiler.Steps.EmitAssembly.OnModule(Module module) at Boo.Lang.Compiler.Steps.EmitAssembly.DefineTypes() at Boo.Lang.Compiler.Steps.EmitAssembly.Run() at Boo.Lang.Compiler.CompilerPipeline.RunStep(CompilerContext context, ICompilerStep step)