diff --git a/win10toast/__init__.py b/win10toast/__init__.py
index 8ed1bc5..770f213 100644
--- a/win10toast/__init__.py
+++ b/win10toast/__init__.py
@@ -153,5 +153,5 @@ def on_destroy(self, hwnd, msg, wparam, lparam):
         Shell_NotifyIcon(NIM_DELETE, nid)
         PostQuitMessage(0)
 
-        return None
+        return 0  # Error fixed (WNDPROC return value cannot be converted to LRESULT TypeError: WPARAM is simple, so must be an int object (got NoneType))