AppSync GraphQL API - MissingSchema: Invalid URL Error despite correct endpoint URL provided

I am calling my AWS AppSync GraphQL API using Python. The API is hosted on AWS amplify. I have entered my AppSync GraphQL endpoint URL correctly.

However I get the following error when I execute a GraphQL query: MissingSchema: Invalid URL 'None': No scheme supplied. Perhaps you meant https://None?

My code is below, the error occurs for the following line client.execute(gql(list_events_query))

def make_client():
    
    headers = {
        'Accept': 'application/json',
        'Content-Type': 'application/json',
    }

    aws = AWSSession(aws_access_key_id=AWS_ACCESS_KEY_ID,
                     aws_secret_access_key=AWS_SECRET_ACCESS_KEY,
                     region_name=AWS_REGION_NAME)
                     
    credentials = aws.get_credentials().get_frozen_credentials()

    auth = AWS4Auth(
        credentials.access_key,
        credentials.secret_key,
        aws.region_name,
        'appsync',
        session_token=credentials.token,
    )

    transport = RequestsHTTPTransport(url=APPSYNC_ENDPOINT),
                                      headers=headers,
                                      auth=auth)
    
    client = Client(transport=transport,
                    fetch_schema_from_transport=True)
    return client

client = make_client()

resp = client.execute(gql(list_events_query))