Fix typo that prevented error message
@@ -369,7 +369,7 @@ class Server(object):
op = msg.pop("op")
except KeyError:
raise ValueError(
- "Received unexpected message without 'op' key: " % str(msg)
+ "Received unexpected message without 'op' key: " + str(msg)
)
if self.counters is not None:
self.counters["op"].add(op)
