--- a/ipc/ipdl/ipdl/type.py
+++ b/ipc/ipdl/ipdl/type.py
@@ -180,17 +180,17 @@ class IPDLType(Type):
     def needsMoreJuiceThan(self, o):
         return not IPDLType.convertsTo(self, o)
 
 class MessageType(IPDLType):
     def __init__(self, nested, prio, sendSemantics, direction,
                  ctor=False, dtor=False, cdtype=None, compress=False,
                  verify=False):
         assert not (ctor and dtor)
-        assert not (ctor or dtor) or type is not None
+        assert not (ctor or dtor) or cdtype is not None
 
         self.nested = nested
         self.prio = prio
         self.nestedRange = (nested, nested)
         self.sendSemantics = sendSemantics
         self.direction = direction
         self.params = [ ]
         self.returns = [ ]