$ionicUser throwing error on Identify

When I call

  $ionicUser.identify({
    user_id: $ionicUser.generateGUID()
  });

The browser spits out the following error:

XMLHttpRequest cannot load https://apps.ionic.io/api/v1/app/(appid)/users/identify. Request header field If-Modified-Since is not allowed by Access-Control-Allow-Headers.

Does anybody know what could be causing this?