diff --git a/tests/cache_config_test.php b/tests/cache_config_test.php
index 068de60..64ceb99 100644
--- a/tests/cache_config_test.php
+++ b/tests/cache_config_test.php
@@ -27,6 +27,13 @@
  */
 class cache_config_test extends \advanced_testcase {
 
+    /**
+     * Temporary directory for loading config into
+     *
+     * @var string
+     */
+    private $tmpdir;
+
     /**
      * We need to load the config files outside of the $CFG->dirroot, so it
      * will be copied out as part of these tests.