diff --git a/notebook/base/zmqhandlers.py b/notebook/base/zmqhandlers.py
index fa0b6f706..542e8ca2a 100644
--- a/notebook/base/zmqhandlers.py
+++ b/notebook/base/zmqhandlers.py
@@ -101,6 +101,7 @@ class WebSocketMixin(object):
     ping_callback = None
     last_ping = 0
     last_pong = 0
+    stream = None
     
     @property
     def ping_interval(self):