我已经安装了Django-Channels，但是在运行daphne-server时出现以下错误：

File "/usr/local/lib/python2.7/dist-packages/channels/asgi.py", line 36, in make_backend
"Cannot import BACKEND %r specified for %s" % (self.configs[name]['BACKEND'], name)

channels.asgi.InvalidChannelLayerError: Cannot import BACKEND 'asgi_redis.RedisChannelLayer' specified for default

我的settings.py是：

CHANNEL_LAYERS = {
"default": {
    "BACKEND": "asgi_redis.RedisChannelLayer",
    "CONFIG": {
        "hosts": [os.environ.get('REDIS_URL', 'redis://X.X.X.X:6379')],
    },
    "ROUTING": "MyProject.routing.channel_routing",
},
}

需要帮助解决此错误。