diff --git a/cache/tests/cache_test.php b/cache/tests/cache_test.php
index 545986ae34a..7f402d56b6c 100644
--- a/cache/tests/cache_test.php
+++ b/cache/tests/cache_test.php
@@ -422,7 +422,7 @@ class cache_phpunit_tests extends advanced_testcase {
             'mode' => cache_store::MODE_APPLICATION,
             'component' => 'phpunit',
             'area' => 'ttltest',
-            'ttl' => -10
+            'ttl' => -86400 // Set to a day in the past to be extra sure.
         ));
         $cache = cache::make('phpunit', 'ttltest');
         $this->assertInstanceOf('cache_application', $cache);
@@ -775,4 +775,4 @@ class cache_phpunit_tests extends advanced_testcase {
         $this->assertTrue($cache->set('test', 'test'));
         $this->assertEquals('test', $cache->get('test'));
     }
-}
\ No newline at end of file
+}