diff --git a/core/modules/simpletest/simpletest.theme.inc b/core/modules/simpletest/simpletest.theme.inc
index 31cbb02..4ee0261 100644
--- a/core/modules/simpletest/simpletest.theme.inc
+++ b/core/modules/simpletest/simpletest.theme.inc
@@ -60,7 +60,7 @@ function theme_simpletest_test_table($variables) {
 
     // Make the class name safe for output on the page by replacing all
     // non-word/decimal characters with a dash (-).
-    $test_class = strtolower(trim(preg_replace("/[^\w\d]/", "-", $key)));
+    $test_class = 'module-' . strtolower(trim(preg_replace("/[^\w\d]/", "-", $key)));
 
     // Select the right "expand"/"collapse" image, depending on whether the
     // category is expanded (at least one test selected) or not.