I believe that both of the assertions should pass, and "returning implicit" should be called twice. Only happens once, instead. **Code: """ returning implicit returning implicit """ class test(IQuackFu): def QuackSet(name as string, obj): pass def QuackGet(name as string): return def QuackInvoke(name as string, *args as (object)): print 'entered invoke' static def op_Implicit(arg as test) as bool: print 'returning implicit' return false first = test() assert not first second as duck = test() assert not second #assertion has FAILED; op_Implicit wasn't called, or went through QuackInvoke **Output: returning implicit