async def METHOD_NAME(opcode):
    assembler = WebsocketFrameAssembler(Mock())
    assembler.message_fetched.set()
    message = re.escape(
        "Websocket put() got a new message when the previous message was "
        "not yet fetched."
    )
    with pytest.raises(ServerError, match=message):
        await assembler.put(Frame(opcode, b""))