How to hide the back button label?

How can I hide the back button label on the nav bar?
I wasn’t able to do it by setting:

Actually the previous doesn’t work even if I set the label. The only thing that works (somehow) is by doing the following:
ion-nav-bar
ion-nav-back-button
Back
/ion-nav-back-button
/ion-nav-bar
… and I don’t want a label in the back button.
I’m using v.1.2.4
Thanks