How to submit the back of the id in iOS

The iOS SDK session object has a postCardId method with an isFront param, but also includes a postBackId method. Which should be used for submitting the back of the id?

Similarly, there are .id and .backId values for the VouchedDetectionMode. I get an error when I try to create VouchedCameraHelper with the backId mode though.

Using the .id mode and postCardId method works for the front of the id, but I get a 500 error when I attempt to submit the back of the id this way.