Couchbase receive Channel null

I am using the latest Couchbase Server with Sync Gateway. On the client application swift I push a document like this:

 let userChannel = "\(self.login)"
    config.channels = [userChannel, "zzzz", "nurse1"]
       config.replicatorType = .pushAndPull
    _pushPullRepl = Replicator.init(config: config)
  _pushPullRepl?.start()

On the server, I receive this document but the problem is that

   "channels": [
        null
      ]

So when I try to pull document by channel I receive 0 document.

I have a configuration file here:

     {
"log": ["*"],
"adminInterface": ":4985",
"databases": {
  "dev": {
    "num_index_replicas": 0,
      "server":"http://localhost:8091",
      "bucket": "dev",
       "username": "admin",
       "password": "adminadmin",
      "users": { 
      "admin": { "disabled": false, "password": "adminadmin"},
      "nurse2": { "disabled": false, "password": "adminadmin", "admin_channels": ["nurse2", "_nurse2","nurse1", "_nurse1"]},
      "nurse1": { "disabled": false, "password": "adminadmin", "admin_channels": ["nurse1", "_nurse1", "nurse2", "_nurse2"] }
    }
  }

}

}

Thank's for your responses.

Edit:

Sync function is

 "sync":
        `function(doc) {channel(doc.channels);}`
      }

and i'm getting error

2019-02-25 18:07:14.234729+0100 CouchbaseLiteTest[2474:1149779] CouchbaseLite Sync ERROR: {Push#1} Got error response to rev -YFzAmAK0VWolQcrIv2DiWH 1-a82becb9eacf1c02d0514aeb7f97cfcef9816bda (seq #4): HTTP 500 'Exception in JS sync function'

i don't have error anymore but channel still null on server :(

I found the solution i was connecting with nurse1 and nurse2 so i have to add function(doc) {channel(doc.channels);} to this users like this

     "users": { 
      "admin": { "disabled": false, "password": "adminadmin"},
      "nurse2": { "disabled": false, "password": "adminadmin","sync":
        `function(doc) {channel(doc.channels);}`, "admin_channels": ["nurse2", "_nurse2","nurse1", "_nurse1"]},
      "nurse1": { "disabled": false, "password": "adminadmin","sync":
        `function(doc) {channel(doc.channels);}`, "admin_channels": ["nurse1", "_nurse1", "nurse2", "_nurse2"] }
    }

Also i had to add channel to document before save it like this

      let channels = MutableArrayObject()
            channels.addString("nurse1")
            mutableDoc.setArray(channels, forKey: "channels")

_________EDIT

sync function on the user level not necessary.