diff I sent to Fernando

IPython/utils/traitlets.py

@@ -978,6 +978,8 b' class List(Instance):'
             args = ((),)
         elif isinstance(default_value, SequenceTypes):
             args = (default_value,)
+        else:
+            raise TypeError('default value of List was %s' % default_value)
 
         super(List,self).__init__(klass=list, args=args, 
                                   allow_none=allow_none, **metadata)