diff --git src/wp-includes/nav-menu.php src/wp-includes/nav-menu.php
index f130d2f3e1..ba1d777bbb 100644
--- src/wp-includes/nav-menu.php
+++ src/wp-includes/nav-menu.php
@@ -92,6 +92,14 @@ function register_nav_menus( $locations = array() ) {
 
 	add_theme_support( 'menus' );
 
+	foreach ( $locations as $key => $value ) {
+		if ( is_int( $key ) ) {
+			$message = __( 'Strings should always be used for menu location slugs.' );
+			_doing_it_wrong( __FUNCTION__, $message, '5.3' );
+			break;
+		}
+	}
+
 	$_wp_registered_nav_menus = array_merge( (array) $_wp_registered_nav_menus, $locations );
 }
 
diff --git tests/phpunit/tests/menu/nav-menu.php tests/phpunit/tests/menu/nav-menu.php
index 2627f9f21a..4577f5e9a9 100644
--- tests/phpunit/tests/menu/nav-menu.php
+++ tests/phpunit/tests/menu/nav-menu.php
@@ -182,6 +182,8 @@ class Tests_Nav_Menu_Theme_Change extends WP_UnitTestCase {
 	/**
 	 * Technically possible to register menu locations numerically.
 	 *
+	 * @expectedIncorrectUsage register_nav_menus
+	 *
 	 * @covers ::wp_map_nav_menu_locations()
 	 */
 	function test_numerical_locations() {
@@ -205,6 +207,8 @@ class Tests_Nav_Menu_Theme_Change extends WP_UnitTestCase {
 	/**
 	 * Technically possible old nav menu locations were registered numerically.
 	 *
+	 * @expectedIncorrectUsage register_nav_menus
+	 *
 	 * @covers wp_map_nav_menu_locations()
 	 */
 	public function test_numerical_old_locations() {