--- a/dom/bindings/Codegen.py
+++ b/dom/bindings/Codegen.py
@@ -748,19 +748,24 @@ class CGAbstractClassHook(CGAbstractStat
     Meant for implementing JSClass hooks, like Finalize or Trace. Does very raw
     'this' unwrapping as it assumes that the unwrapped type is always known.
     """
     def __init__(self, descriptor, name, returnType, args):
         CGAbstractStaticMethod.__init__(self, descriptor, name, returnType,
                                         args)
 
     def definition_body_prologue(self):
-        return """
+        if self.descriptor.nativeOwnership == 'nsisupports':
+            assertion = ('  MOZ_STATIC_ASSERT((IsBaseOf<nsISupports, %s>::value), '
+                         '"Must be an nsISupports class");') % self.descriptor.nativeType
+        else:
+            assertion = ''
+        return """%s
   %s* self = UnwrapDOMObject<%s>(obj);
-""" % (self.descriptor.nativeType, self.descriptor.nativeType)
+""" % (assertion, self.descriptor.nativeType, self.descriptor.nativeType)
 
     def definition_body(self):
         return self.definition_body_prologue() + self.generate_code()
 
     def generate_code(self):
         # Override me
         assert(False)