diff --git a/Lib/unittest/case.py b/Lib/unittest/case.py
index c0170d1..758924d 100644
--- a/Lib/unittest/case.py
+++ b/Lib/unittest/case.py
@@ -519,7 +519,7 @@
         case as failed but resumes execution at the end of the enclosed
         block, allowing further test code to be executed.
         """
-        if not self._outcome.result_supports_subtests:
+        if self._outcome is None or not self._outcome.result_supports_subtests:
             yield
             return
         parent = self._subtest
diff --git a/Lib/unittest/test/test_case.py b/Lib/unittest/test/test_case.py
index 6b34397..6d58201 100644
--- a/Lib/unittest/test/test_case.py
+++ b/Lib/unittest/test/test_case.py
@@ -425,6 +425,20 @@
         expected = ['a1', 'a2', 'b1']
         self.assertEqual(events, expected)
 
+    def test_subtests_debug(self):
+        # Test debug() with a test that uses subTest() (bpo-34900)
+        events = []
+
+        class Foo(unittest.TestCase):
+            def test_a(self):
+                events.append('test case')
+                with self.subTest():
+                    events.append('subtest 1')
+
+        Foo('test_a').debug()
+
+        self.assertEqual(events, ['test case', 'subtest 1'])
+
     # "This class attribute gives the exception raised by the test() method.
     # If a test framework needs to use a specialized exception, possibly to
     # carry additional information, it must subclass this exception in