diff --git a/Lib/test/test_support.py b/Lib/test/test_support.py
index 0eb74c2..44d4b24 100644
--- a/Lib/test/test_support.py
+++ b/Lib/test/test_support.py
@@ -251,7 +251,7 @@
     """Run tests from unittest.TestCase-derived classes."""
     suite = unittest.TestSuite()
     for cls in classes:
-        if isinstance(cls, unittest.TestCase):
+        if isinstance(cls, (unittest.TestSuite, unittest.TestCase)):
             suite.addTest(cls)
         else:
             suite.addTest(unittest.makeSuite(cls))