diff --git a/src/if_py_both.h b/src/if_py_both.h
index 7f38e17..6d71ac1 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -2043,7 +2043,7 @@
 	    return NULL;
 
 	VimTryStart();
-	dict_extend(self->dict, tv.vval.v_dict, (char_u *) "force");
+	dict_extend(self->dict, tv.vval.v_dict, (char_u *) "force", NULL);
 	clear_tv(&tv);
 	if (VimTryEnd())
 	    return NULL;