AngularFire - TypeError: Cannot read property '$add' of undefined

I use Ionic and AngularFire for make a chat, but I have problem: the following error

TypeError: Cannot read property '$add' of undefined at Scope.$scope.sendChat (controllers.js:12)

prevents me to advance in my work. I don't understand where I can found the solution.

This is my code in controller.JS:

angular.module('starter.controllers', ['firebase'])

.factory("Chats", function($firebaseArray) {
  var itemsRef = new Firebase("http://ift.tt/1NquD98");
  return $firebaseArray(chatsRef);
})

.controller('ChatsCtrl', ['$scope', '$rootScope', 
  function($scope, Chats, $rootScope) {

  $scope.sendChat = function(chat) {
      $scope.chats.$add({
        user: 'guest',
        message: chat.message

      });

      chat.message = "";

    };

}])

This is my code in HTML :

<ion-view view-title="Chats">
  <ion-content>
    <div class="list">
      <a class="item item-avatar">
        <img>
        <h2>{{ chat.user }}</h2>
        <p>{{ data.text }}</p>
      </a>
    </div>
    <form>
      <div class="list">
        <div class="item item-input-inset">
          <label class="item-input-wrapper">
            <input type="text" placeholder="Message" ng-model="chat.message">
          </label>
          <button class="button button-assertive ion ion-ios-chatbubble" ng-click="sendChat(chat)"></button>
        </div>
      </div>
    </form>
  </ion-content>
</ion-view>

Can I found a charitable soul to help me?