iOS: SwiftyJSON parsing from Alamofire is an array. Getting null when trying to parse

the api request returns something like this:

[  
   {  
  "macAddress":"x:x:x:x",
  "hatInfo":{  
     "hatHierarchyString":"hello",
     "floorRefId":"xxx",
     .....

I am using swiftyJSON to parse this, I got data from Alamofire request:

let json = JSON(data)
let mapHeir = json[0]["hatInfo"]["hatHierarchyString"]
print(mapHeir)

This returns null, I tried:

let mapHeir = json[0]["hatInfo"]["hatHierarchyString"].string
let mapHeir = json[0]["hatInfo"]["hatHierarchyString"].stringValue
let mapHeir = json["hatInfo"]["hatHierarchyString"]

and all of them return empty. How can I get to hatHierarchyString?