next_workspace and F20

[Felix Roeser]

Hi All,

i have trouble setting up next- and prev_workspace_key.

In my .Xmodmap i set 
keycode 234 = F19
keycode 233 = F20
for the two navigation keys on my thinkpad keyboard. Not i want to
switch desktops with F19 and F20. But 
next_workspace_key=F20
prev_workspace_key=F19
is not working :( When i combine F19 with the Alt key it is working.
Is it not possible to assign a single key to this function?

Thanks for your tipps

-- 
Bye
Felix