diff --git a/tests/phpunit/tests/cache.php b/tests/phpunit/tests/cache.php
index 42fdac9d2de15..a21d672938ebf 100644
--- a/tests/phpunit/tests/cache.php
+++ b/tests/phpunit/tests/cache.php
@@ -57,7 +57,7 @@ public function test_is_valid_key( $key, $valid ) {
 	 *     @type bool  $valid Whether the key should be considered valid.
 	 * }
 	 */
-	public function data_is_valid_key() {
+	public static function data_is_valid_key() {
 		return array(
 			'false'          => array( false, false ),
 			'null'           => array( null, false ),