diff --git a/libbeat/scripts/generate_index_pattern.py b/libbeat/scripts/generate_index_pattern.py
index 8bd2355f46e..6efa6d78ab5 100644
--- a/libbeat/scripts/generate_index_pattern.py
+++ b/libbeat/scripts/generate_index_pattern.py
@@ -35,7 +35,7 @@ def field_to_json(desc, path, output):
     global unique_fields
 
     if path in unique_fields:
-        print "ERROR: Field", path, "is duplicated. Please delete it and try again. Fields already are", unique_fields
+        print("ERROR: Field {} is duplicated. Please delete it and try again. Fields already are {}".format(path, ", ".join(unique_fields)))
         sys.exit(1)
     else:
         unique_fields.append(path)