diff --git a/src/trans-websocket.coffee b/src/trans-websocket.coffee
index 7712f62f..57716faf 100644
--- a/src/trans-websocket.coffee
+++ b/src/trans-websocket.coffee
@@ -101,7 +101,8 @@ class WebSocketReceiver extends transport.GenericReceiver
             super
 
     heartbeat_timeout: ->
-        @session.close(3000, 'No response from heartbeat')
+        if @session?
+            @session.close(3000, 'No response from heartbeat')