[Grml] making zsh beep like bash

When using the bash shell, backspacing on an empty command line causes a beep. How would one make zsh behave that way?

I have setopt beep in my setup files, and the ambiguous completion of filenames beeps as expected, but not the backspacing on an empty command line. I find no information in the man pages on this matter.

Any suggestion most welcome.

Chuck