diff --git a/kafka/consumer/subscription_state.py b/kafka/consumer/subscription_state.py
index 9e7f080..19046ae 100644
--- a/kafka/consumer/subscription_state.py
+++ b/kafka/consumer/subscription_state.py
@@ -225,7 +225,7 @@ class SubscriptionState(object):
 
         for tp in assignments:
             if tp.topic not in self.subscription:
-                raise ValueError("Assigned partition %s for non-subscribed topic." % tp)
+                raise ValueError("Assigned partition %s for non-subscribed topic." % str(tp))
         self.assignment.clear()
         for tp in assignments:
             self._add_assigned_partition(tp)