diff --git a/gql/transport/phoenix_channel_websockets.py b/gql/transport/phoenix_channel_websockets.py
index 27e58f2a..26ab159b 100644
--- a/gql/transport/phoenix_channel_websockets.py
+++ b/gql/transport/phoenix_channel_websockets.py
@@ -34,6 +34,7 @@ def __init__(
         """
         self.channel_name = channel_name
         self.heartbeat_interval = heartbeat_interval
+        self.heartbeat_task: Optional[asyncio.Future] = None
         self.subscription_ids_to_query_ids: Dict[str, int] = {}
         super(PhoenixChannelWebsocketsTransport, self).__init__(*args, **kwargs)
 
diff --git a/gql/transport/websockets.py b/gql/transport/websockets.py
index 7e26f31c..2dce0545 100644
--- a/gql/transport/websockets.py
+++ b/gql/transport/websockets.py
@@ -175,8 +175,9 @@ async def _receive(self) -> str:
         """Wait the next message from the websocket connection and log the answer
         """
 
-        # We should always have an active websocket connection here
-        assert self.websocket is not None
+        # It is possible that the websocket has been already closed in another task
+        if self.websocket is None:
+            raise TransportClosed("Transport is already closed")
 
         # Wait for the next websocket frame. Can raise ConnectionClosed
         data: Data = await self.websocket.recv()
@@ -387,6 +388,8 @@ async def _receive_data_loop(self) -> None:
                 except (ConnectionClosed, TransportProtocolError) as e:
                     await self._fail(e, clean_close=False)
                     break
+                except TransportClosed:
+                    break
 
                 # Parse the answer
                 try:
diff --git a/tests/test_phoenix_channel_subscription.py b/tests/test_phoenix_channel_subscription.py
index ef46db47..16ce5c34 100644
--- a/tests/test_phoenix_channel_subscription.py
+++ b/tests/test_phoenix_channel_subscription.py
@@ -101,9 +101,13 @@ async def stopping_coro():
 @pytest.mark.parametrize("server", [server_countdown], indirect=True)
 @pytest.mark.parametrize("subscription_str", [countdown_subscription_str])
 async def test_phoenix_channel_subscription(event_loop, server, subscription_str):
+    import logging
     from gql.transport.phoenix_channel_websockets import (
         PhoenixChannelWebsocketsTransport,
     )
+    from gql.transport.websockets import log as websockets_logger
+
+    websockets_logger.setLevel(logging.DEBUG)
 
     path = "/graphql"
     url = f"ws://{server.hostname}:{server.port}{path}"