diff --git a/contrib/byteify-strings.py b/contrib/byteify-strings.py
--- a/contrib/byteify-strings.py
+++ b/contrib/byteify-strings.py
@@ -227,4 +227,7 @@
                 process(fin, fout, opts)
 
 if __name__ == '__main__':
+    if sys.version_info.major < 3:
+        print('This script must be run under Python 3.')
+        sys.exit(3)
     main()