diff --git a/py/makeqstrdefs.py b/py/makeqstrdefs.py
index 69aaefb3e6..92a19c3920 100644
--- a/py/makeqstrdefs.py
+++ b/py/makeqstrdefs.py
@@ -11,7 +11,7 @@ import os
 
 # Blacklist of qstrings that are specially handled in further
 # processing and should be ignored
-QSTRING_BLACK_LIST = {'NULL', 'number_of', }
+QSTRING_BLACK_LIST = set(['NULL', 'number_of'])
 
 
 def write_out(fname, output):