diff --git a/websocket/_app.py b/websocket/_app.py
index e5efe66..94fc697 100644
--- a/websocket/_app.py
+++ b/websocket/_app.py
@@ -27,6 +27,7 @@ import time
 import traceback
 import sys
 import select
+import six
 
 
 from ._core import WebSocket, getdefaulttimeout, logger