Index: src/wp-admin/includes/plugin.php
===================================================================
--- src/wp-admin/includes/plugin.php	(revision 46434)
+++ src/wp-admin/includes/plugin.php	(working copy)
@@ -1373,7 +1373,7 @@
 	}
 
 	$new_sub_menu = array( $menu_title, $capability, $menu_slug, $page_title );
-	if ( null === $position ) {
+	if ( ! is_int( $position ) ) {
 		$submenu[ $parent_slug ][] = $new_sub_menu;
 	} else {
 		// If position is equal or higher than the number of items in the array, append the submenu.