How to execute a stunned zombie in Dying Light?

I'm assuming that when you hit a zombie and then see its skeleton (X-ray vision) it means it's stunned, but how do I then execute it?

There's no "hint" on the screen or anything to tell me what key to hit. There's nothing in the key binding settings about execution. I tried E (kick) and LMB (attack), but they do not work.

I'm playing on PC.