diff --git a/cowrie/output/elasticsearch.py b/cowrie/output/elasticsearch.py
index 60a8f44..ec5c81b 100644
--- a/cowrie/output/elasticsearch.py
+++ b/cowrie/output/elasticsearch.py
@@ -26,7 +26,7 @@ class Output(cowrie.core.output.Output):
         pass
 
     def write(self, logentry):
-        for i in logentry.keys():
+        for i in list(logentry.keys()):
             # remove twisted 15 legacy keys
             if i.startswith('log_'):
                 del logentry[i]