diff --git a/panda/src/display/windowProperties.I b/panda/src/display/windowProperties.I
index a5682ab0e8..c86cb0b204 100644
--- a/panda/src/display/windowProperties.I
+++ b/panda/src/display/windowProperties.I
@@ -831,7 +831,7 @@ has_parent_window() const {
 INLINE void WindowProperties::
 clear_parent_window() {
   _specified &= ~S_parent_window;
-  _parent_window=NULL;
+  _parent_window = (size_t)NULL;
 }