1: define a class that implements IQuackFu 2: derive a subclass 3: call super in the subclass constructor The runtaime fails with a casting exception. This seems to be because IQuackFu has redirected the call to QuackInvoke. The following reproduces the problem: import System class base(IQuackFu): def constructor(s as String): pass def QuackGet(s as String): pass def QuackSet(s as String,v as Object): pass def QuackInvoke(s as String,args as (Object)): pass class derv(base): def constructor(): super("crunch") derv()