diff --git a/websocket/_abnf.py b/websocket/_abnf.py
index b286c43..231277b 100644
--- a/websocket/_abnf.py
+++ b/websocket/_abnf.py
@@ -108,7 +108,7 @@ class ABNF(object):
             raise WebSocketProtocolException("rsv is not implemented, yet")
 
         if self.opcode not in ABNF.OPCODES:
-            raise WebSocketProtocolException("Invalid opcode " + self.opcode)
+            raise WebSocketProtocolException("Invalid opcode " + str(self.opcode))
 
         if self.opcode == ABNF.OPCODE_PING and not self.fin:
             raise WebSocketProtocolException("Invalid ping frame.")