diff --git a/lib/tests/rtlcss_test.php b/lib/tests/rtlcss_test.php
index 46e01f13736fc..55c7ecb297530 100644
--- a/lib/tests/rtlcss_test.php
+++ b/lib/tests/rtlcss_test.php
@@ -1129,7 +1129,7 @@ protected function assert_sample($data, $output = null) {
      * @param array $data the provider data.
      * @dataProvider background_image_provider
      */
-    public function test_background_image($data) {
+    public function test_background_image($data): void {
         $this->markTestSkipped('Not yet supported!');
         $output = new OutputFormat();
         $this->assert_sample($data, $output);
@@ -1196,7 +1196,7 @@ public function test_properties($data): void {
      * @param array $data the provider data.
      * @dataProvider special_provider
      */
-    public function test_special($data) {
+    public function test_special($data): void {
         $this->markTestSkipped('Not yet supported!');
         $output = new OutputFormat();
         $output->set('SpaceBeforeRules', ' ');
@@ -1224,7 +1224,7 @@ public function test_transform_origin($data): void {
      * @param array $data the provider data.
      * @dataProvider transforms_provider
      */
-    public function test_transforms($data) {
+    public function test_transforms($data): void {
         $this->markTestSkipped('Not yet supported!');
         $output = new OutputFormat();
         $output->set('SpaceBeforeRules', ' ');