diff --git a/moo/moopython/pygtk/moocommand.defs b/moo/moopython/pygtk/moocommand.defs
index 45e680e3..838a635e 100644
--- a/moo/moopython/pygtk/moocommand.defs
+++ b/moo/moopython/pygtk/moocommand.defs
@@ -66,7 +66,7 @@
     '("const-char*" "name")
     '("const-char*" "display_name")
     '("MooCommandType*" "type")
-    '("strv" "data_keys")
+    '("strv" "data_keys" (null-ok))
   )
 )