diff --git a/src/python/grpcio/grpc/experimental/aio/_channel.py b/src/python/grpcio/grpc/experimental/aio/_channel.py
index 9e9c7529f34..536206241fe 100644
--- a/src/python/grpcio/grpc/experimental/aio/_channel.py
+++ b/src/python/grpcio/grpc/experimental/aio/_channel.py
@@ -395,7 +395,7 @@ class Channel:
         # A new set is created acting as a shallow copy because
         # when cancellation happens the calls are automatically
         # removed from the originally set.
-        calls = frozenset(self._ongoing_calls.calls)
+        calls = WeakSet(self._ongoing_calls.calls)
         for call in calls:
             if call is not None:
                 call.cancel()
diff --git a/src/python/grpcio_tests/tests_aio/unit/metadata_test.py b/src/python/grpcio_tests/tests_aio/unit/metadata_test.py
index db32add303a..961ec29fceb 100644
--- a/src/python/grpcio_tests/tests_aio/unit/metadata_test.py
+++ b/src/python/grpcio_tests/tests_aio/unit/metadata_test.py
@@ -210,6 +210,8 @@ class TestMetadata(AioTestBase):
         self.assertEqual(_RESPONSE, await call)
         self.assertEqual(grpc.StatusCode.OK, await call.code())
 
+    @unittest.skipIf(platform.system() == 'Windows',
+                     'https://github.com/grpc/grpc/issues/21943')
     async def test_invalid_metadata(self):
         multicallable = self._client.unary_unary(_TEST_CLIENT_TO_SERVER)
         for exception_type, metadata in _INVALID_METADATA_TEST_CASES: