Ember RSVP.Promise resolve function return undefined

I have function in my adapter "findQuery" the function need to GET data from the server and return it.

the function:

findQuery: function(store, type, query) {
  var userId = query['user_id'];
  delete query['user_id'];

  if (this.sortQueryParams) {
    query = this.sortQueryParams(query);
  }

  var adapter = this;
  // Create nested url
  //return this.ajax(this.urlPrefix(this.pathForType(type.typeKey), this.buildURL(type, userId)), 'GET', { data: query });
  // We would like to relate the rows to the connected user
  return new Ember.RSVP.Promise(function(resolve, reject) {
    adapter.ajax(adapter.buildURL(type.typeKey, null, userId), 'GET', { data: query }).then(function(devices) {
      store.find('user', userId).then(function(user) {
        devices.setEach('user_id', user);
        resolve(devices);  // devices is set exactly as i wanted!
      }, function(failed) {
        reject(failed);
      });

    }, function(error) {
      reject(error);
    });
  });
}

so when I debug the function Ii see that the function get into the resolve with the correct data (the API returned all the requested data as Array of objects). And then I get log Error:

Error: Assertion Failed: The response from a findQuery must be an Array, not undefined

What could be the issue?