Selfwritten widget won't bind to keymap

["xRaich[o]²x" <raichoo@xxxxxxxxxxxxxx>]

Hi,
I'm having the following problem. I'm trying to do something like this:

function tester()
{
	zle undo
	zle accept-line
}

zle -N tester
bindkey -M menuselect "i" tester

This won't bind to the desired key (it lists in the keybindings but doesn't 
execute when pressing the i-key) , but doing something like

bindkey -M menuselect "u" undo

works.
I find this VERY confusing. What am i doing wrong?

Regards,
Björn