diff --git a/drupal_apc_cache.inc b/drupal_apc_cache.inc
index 2f7e330..9ad15f3 100644
--- a/drupal_apc_cache.inc
+++ b/drupal_apc_cache.inc
@@ -91,7 +91,7 @@ class DrupalAPCCache implements DrupalCacheInterface {
     }
 
     // When we are in testing mode we add the test prefix.
-    if ($test_prefix = drupal_valid_test_ua()) {
+    if ($test_prefix = drupal_valid_test_ua($_SERVER['HTTP_USER_AGENT'])) {
       $prefix = $test_prefix . '::' . $prefix;
     }
     else if (isset($GLOBALS['drupal_test_info'])) {