Add a debugging traceback.

lib/interface.py

@@ -243,6 +243,8 @@ class Interface:
 
     def queue_request(self, request):
         '''Queue a request.'''
+        if type(request) != type({}):
+            traceback.print_exc(file=sys.stderr)
         self.request_time = time.time()
         self.unsent_requests.append(request)