The example below fails to compile with the following error... Error BCE0055: Internal compiler error: Unable to cast object of type 'Boo.Lang.Compiler.Ast.CallableTypeReference' to type 'Boo.Lang.Compiler.Ast.SimpleTypeReference'.. namespace test import System import System.Collections.Generic as gen class foo: def Test(val as int): print val.ToString() def RunTest(): steps = gen.Dictionary[of int, callable(int)]() steps.Add(1, Test) steps.Add(5, Test) for item as gen.KeyValuePair[of int, callable(int)] in steps: item.Value(item.Key)