rxvt-unicode depth ignored

[[LoN]Kamikaze]

Hallo,

I'm running

rxvt-unicode (urxvt) v7.7 - released: 2006-02-21

under FreeBSD and the URxvt.depth ressource in my .Xdefaults is ignored.
Only if I run
# urxvt -depth 32
the resource is set properly. How can I force it to honour the setting
in my .Xdefaults file?