Invalid type in JSON write in Swift Alamofire

I am having issues sending my parameters over Alamofire. This is the JSON that the server expects:

{
“users”:[{“_id”:”1234567”}, {“_id”:”665543”}]
}

Now, in my Swift code, this is how I’m trying to build out the above object:

let parameters = [“users” : users]
return sendRequest(.PUT, url: “example.com", parameters: parameters)

where users is an array. Here is more info on what users is (I have taken out some of the other inherits to simplify):

class User : MyBase, CustomStringConvertible {
  var name: String

  var description : String {
     get{
          return ["_id" : self._id!].description
     }
 }
}

class MyBase {
  var _id: String?
}

If I do a debugPrint(users), this is what I get:

[“users”: [["_id": "1234567"], ["_id": "665543"]]]

And here’s the sendRequest method that does the Alamofire stuff:

static func sendRequest(method: Alamofire.Method, url: String, parameters: [String: AnyObject]?) {
        Alamofire.request(method, url, parameters: parameters as! [String: [User]], encoding: .JSON)
            .validate()
            .responseObject { (response: Response<User, NSError>) in
                switch response.result {
                case .Success(let value):
                    // do something
                case .Failure(let error):
                    // do something else
                }
        }
    }

And the error it throws on the Alamofire.request line is this:

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Invalid type in JSON write (MyApp.User)’

Ultimately, what I have is an array of user objects, and I need to send that to my server in the JSON format mentioned above. Being new to Swift, I’ve been struggling trying to figure this out, so any help is appreciated.