Refresh first firebase field

// Initialize Firebase
var config = {
    databaseURL: "https://visit-hotels.firebaseio.com",
};
firebase.initializeApp(config);
var app = angular.module("nowtimereal", ["firebase"]);

// this factory returns a synchronized array of nowusers
app.factory("peopleNow", ["$firebaseArray",
  function($firebaseArray) {
    // create a reference to the database location where we will store our data
    var ref = firebase.database().ref('nowusers');

    // this uses AngularFire to create the synchronized array
    return $firebaseArray(ref);
  }
]);

app.controller("NowCtrl", ["$scope", "peopleNow", function($scope, nowPeople) {

   $scope.now = nowPeople;
   $scope.user = Math.round(Math.random() * 25);
   $scope.now.$save({
          count: $scope.user
    });

}]);

I have the following code, I do not know if I'm doing it wrong or it's a problem, but I want to update the first item in the database.

{
  "nowusers" : {
    "-L2LfbYxbwXDXiWNpt27" : {
      "count" : 13
    }
  }
}

I am using angularfire with the method $ save, but it does not work for me I have the following error

Possibly unhandled rejection: Invalid record; could not determine key for [object Object]

Any help? Thank you very much