Imagemagick import utility hangs

Hi.

I have a virtual server running freebsd, and I have installed X11 + Xvfb + ImageMagick on it. I have managed to start the X server and take a screenshot of it, with the following incantations:

startx -- /usr/local/bin/Xvfb :99 -ac -screen 0 1280x1024x24 &
DISPLAY=:99 import -window root screenshot.png

Which gives me a nice looking screenshot. The contents of my ~/.xinitrc is as follows:

#!/bin/sh
# $Xorg: xinitrc.cpp,v 1.3 2000/08/17 19:54:30 cpqbld Exp $

userresources=$HOME/.Xresources
usermodmap=$HOME/.Xmodmap
sysresources=/usr/local/lib/X11/xinit/.Xresources
sysmodmap=/usr/local/lib/X11/xinit/.Xmodmap

# merge in defaults and keymaps

if [ -f $sysresources ]; then
    xrdb -merge $sysresources
fi

if [ -f $sysmodmap ]; then
    xmodmap $sysmodmap
fi

if [ -f $userresources ]; then
    xrdb -merge $userresources
fi

if [ -f $usermodmap ]; then
    xmodmap $usermodmap
fi

twm &
exec xterm -geometry 80x66+0+0 -name login

Now, this works fine and all, but if I try to start a program up in the X session before calling import, it just hangs. Eg. If I call:

DISPLAY=:99 xclock -geometry 500x500 &
DISPLAY=:99 import -window root screenshot.png

.. then import never returns.

What could be the problem?