diff --git a/aiokafka/cluster.py b/aiokafka/cluster.py
index d0d64418..fb67699b 100644
--- a/aiokafka/cluster.py
+++ b/aiokafka/cluster.py
@@ -56,6 +56,7 @@ def __init__(self, **configs):
         self.need_all_topic_metadata = False
         self.unauthorized_topics = set()
         self.internal_topics = set()
+        self.external_topics = set()
         self.controller = None
 
         self.config = copy.copy(self.DEFAULT_CONFIG)
@@ -193,7 +194,7 @@ def topics(self, exclude_internal_topics=True):
             set: {topic (str), ...}
         """
         if exclude_internal_topics:
-            return self.external_topics 
+            return self.external_topics
         else:
             return self._topics