unset backgrounds with bg_pixmap[NORMAL] = "" or what else ?

[Alexander "Werth (hrzpub)" <awerth hrzpub tu-darmstadt de>]

Hy,
I have created a combined pixmap/xenophilia theme and at some places I
need to unset the gtk background pixmap.
I have a bg_pixmap[NORMAL] = "background.xpm" in the theme file and need
to unset this.
Curently I'm doing this with: bg_pixmap[NORMAL]    = "" and it seam to
work but this causes the following error message:
/convert: Datei oder Verzeichnis nicht gefunden
/convert: Datei oder Verzeichnis nicht gefunden
gdk_imlib ERROR: Cannot load image: /home/alex/.themes/AquaXen-NT/gtk/
All fallbacks failed.
There must be a way to unset this bg without getting this error message.
Worst of all this caused some apps to crash. I can't find an example how
to do this in any of the other themes.
Thanks in advance,
Alexander Werth