diff --git a/src/wp-admin/includes/plugin.php b/src/wp-admin/includes/plugin.php
index 32a37f106a..30a37d5ad9 100644
--- a/src/wp-admin/includes/plugin.php
+++ b/src/wp-admin/includes/plugin.php
@@ -1389,7 +1389,7 @@ function add_submenu_page( $parent_slug, $page_title, $menu_title, $capability,
 		$submenu[ $parent_slug ][] = $new_sub_menu;
 	} else {
 		// If position is equal or higher than the number of items in the array, append the submenu.
-		if ( $position >= count( $submenu[ $parent_slug ] ) ) {
+		if ( is_countable( $submenu[ $parent_slug ] ) && $position >= count( $submenu[ $parent_slug ] ) ) {
 			$submenu[ $parent_slug ][] = $new_sub_menu;
 		} else {
 			// Test for a negative position.