diff --git a/core/modules/simpletest/src/TestDiscovery.php b/core/modules/simpletest/src/TestDiscovery.php
index bb8867d..1510bb4 100644
--- a/core/modules/simpletest/src/TestDiscovery.php
+++ b/core/modules/simpletest/src/TestDiscovery.php
@@ -525,7 +525,10 @@ public static function getPhpunitTestSuite($classname, $is_core_class = FALSE) {
         return $matches[2];
       }
       else {
-        // We allow non-standard test suites in Contrib.
+        // All legal Drupal tests must have a non-empty value for a test suite.
+        // In order to allow Contrib modules to use non-standard suites, we
+        // supply a default value of 'Unclassified' for these tests.
+        // @see TestDiscovery::getTestSuite().
         if (!$is_core_class) {
           return 'Unclassified';
         }