[SyntaxError: Failed to parse JSON body: Unexpected token <]

[ghost]

I have the following piece of code.

// Method to generate manager with Q promises
function getFcmManager (socket) {
  // Define a client service constructor
  REST = restler.service(function (socket) {
    this.defaults.query = {
      // Query parameters here
    };
  }, {
    baseURL: API_PATH
  }, {
    POSTNOTIFICATION: function(object) {
      return this.post('send', {
        parser: restler.parsers.json,
        data: JSON.stringify(object),
        headers: {
          'Authorization': 'key=' + API_KEY,
          'Content-Type': 'application/json'
        }
      });
    }
  });
  // Create a client
  var rest = new REST(socket);
  // FCM manager
  var fcm = {
    // Post notification
    postnotification: function (token) {
      return Q.Promise(function (resolve, reject) {
        var settings = {
          notification: {
            title: "Notification title",  // Any value
            body: "Notification body",  // Any value
            sound: "default", // If you want notification sound
            click_action: "FCM_PLUGIN_ACTIVITY",  // Must be present for Android
            icon: "fcm_push_icon"  // White icon Android resource
          },
          data:{
            param1: "value1",  // Any data to be retrieved in the notification callback
            param2: "value2"
          },
          to: token, // Topic or single device
          priority: "high", // If not set, notification won't be delivered on completely closed iOS app
          restricted_package_name: "" // Optional. Set for application filtering
        }
        rest.POSTNOTIFICATION(settings).on('complete', function (data) {
          console.log('POSTNOTIFICATION', data);
          resolve(data);
        });
      });
    }
  }
  // Return
  return fcm;
}

And when I use:
fcm.postnotification(token);

It throws:
POSTNOTIFICATION [SyntaxError: Failed to parse JSON body: Unexpected token <]