Return a 404 error when no fallback templates are found

src/wp-includes/rest-api/endpoints/class-wp-rest-templates-controller.php

@@ -153,6 +153,7 @@ public function register_routes() {
 	 *
 	 * @since 6.1.0
 	 * @since 6.3.0 Ignore empty templates.
+	 * @since 6.5.3 Return a 404 error if no fallback templates are found.
 	 *
 	 * @param WP_REST_Request $request The request instance.
 	 * @return WP_REST_Response|WP_Error
@@ -165,6 +166,10 @@ public function get_template_fallback( $request ) {
 			array_shift( $hierarchy );
 		} while ( ! empty( $hierarchy ) && empty( $fallback_template->content ) );
 
+		if ( ! $fallback_template ) {
+			return new WP_Error( 'rest_template_not_found', __( 'No fallback templates exist for that slug.' ), array( 'status' => 404 ) );
+		}
+
 		$response = $this->prepare_item_for_response( $fallback_template, $request );
 
 		return rest_ensure_response( $response );

tests/phpunit/tests/rest-api/wpRestTemplatesController.php

@@ -909,6 +909,18 @@ public function test_get_template_fallback() {
 		$this->assertSame( 'index', $response->get_data()['slug'], 'Should fallback to `index.html` when  ignore_empty is `true`.' );
 	}
 
+	/**
+	 * @ticket 60909
+	 * @covers WP_REST_Templates_Controller::get_template_fallback
+	 */
+	public function test_get_template_fallback_not_found() {
+		wp_set_current_user( self::$admin_id );
+		$request = new WP_REST_Request( 'GET', '/wp/v2/templates/lookup' );
+		$request->set_param( 'slug', 'not-found' );
+		$response = rest_get_server()->dispatch( $request );
+		$this->assertErrorResponse( 'rest_template_not_found', $response, 404 );
+	}
+
 	/**
 	 * @ticket 57851
 	 *