diff --git a/core/modules/simpletest/simpletest.install b/core/modules/simpletest/simpletest.install
index 3a57dd4..df0b4ca 100644
--- a/core/modules/simpletest/simpletest.install
+++ b/core/modules/simpletest/simpletest.install
@@ -138,7 +138,7 @@ function simpletest_schema() {
     ),
     'primary key' => array('message_id'),
     'indexes' => array(
-      'reporter' => array('test_class', 'message_id'),
+      'reporter' => array(array('test_class', 255), 'message_id'),
     ),
   );
   $schema['simpletest_test_id'] = array(