interface IFoo[of T]: def Bar(ref i as T) or class Foo[of T]: def Bar(ref i as T): pass causes the following compiler error... Entering Boo.Lang.Compiler.Steps.EmitAssembly ASSEMBLY LOADED: <dynamic>test, Version=1.0.2957.0, Culture=neutral, PublicKeyToken=null ASSEMBLY LOADED: C:\WINDOWS\assembly\GAC_32\ISymWrapper\2.0.0.0__b03f5f7f11d50a3a\ISymWrapper.dll Leaving Boo.Lang.Compiler.Steps.EmitAssembly BOOC : error BCE0011: An error occurred during the execution of the step 'Boo.Lang.Compiler.Steps.EmitAssembly': 'Object reference not set to an instance of an object.'. at Boo.Lang.Compiler.Steps.EmitAssembly.GetReferenceTypeName(Type t) at Boo.Lang.Compiler.Steps.EmitAssembly.GetParameterTypes(ParameterDeclarationCollection parameters) at Boo.Lang.Compiler.Steps.EmitAssembly.DefineMethod(TypeBuilder typeBuilder, Method method, MethodAttributes attributes) at Boo.Lang.Compiler.Steps.EmitAssembly.DefineTypeMembers(TypeDefinition typeDefinition) at Boo.Lang.Compiler.Steps.EmitAssembly.DefineTypes() at Boo.Lang.Compiler.Steps.EmitAssembly.Run() at Boo.Lang.Compiler.CompilerPipeline.RunStep(CompilerContext context, ICompilerStep step) — End of inner exception stack trace —