asyncio.await失败,返回TypeError:无法解压缩不可iterable coroutine对象

我有这个密码:

done, pending = asyncio.wait(
    [a, b],
    return_when=asyncio.FIRST_COMPLETED)

但它失败了:

Traceback (most recent call last):
  ...
  File "/.../api.py", line 83, in websockets_handler
    return_when=asyncio.FIRST_COMPLETED)
TypeError: cannot unpack non-iterable coroutine object