[STUMP] add-head crashes

Since the changes in f168e3b7116c106ded81343f07ebd29bc784f2a0 (I can
only assume that's it), stump crashes when I add an extra screen. I'm
using stump on X with no DE, and adding the screen with this script,
bound to a keypress:

#!/bin/sh
xrandr --output LVDS1 --mode 1280x800 --pos 1440x100 --rotate normal --output DP1 --off --output VGA1 --mode 1440x900 --pos 0x0 --rotate normal

This used to work all right, but now crashes stumpwm. I can't copy the
actual backtrace, since copy-unhandled-error doesn't seem to actually
copy it in this case, but here's the top few lines. At the time of the
crash, I had two groups, one with a single frame, and the other with two
frames (one of the frames empty, the other with Pidgin's "Buddy List" in
it).

The value
  (#S(frame 0 #S(TILE-WINDOW "Buddy List" #x1200056) 0 0 640 800)
   #S(frame 1 nil 640 800))
is not of type
  STUMPWM::FRAME.

Backtrace for: etc
0: ((:METHOD GROUP-ADD-HEAD (TILE-GROUP t)) #<TILE-GROUP {etc}> #S(frame 1 NIL 0 0 1440 900)) [fast-method]
1: (ADD-HEAD #S<screen #<XLIB:SCREEN :0.0 1280x800x24 TRUE-COLOR>> #S(frame 1 nil 0 0 1440 900))
2: (SCALE-SCREEN #S<screen #<XLIB:SCREEN :0.0 1280x800x24 TRUE-COLOR>> (#S(frame 0 NIL 1440 800) #S(frame 1 NIL 0 0 1440 900)))

So, whatever's going on, stump isn't expecting two frames in the group
being added, but I really don't know how this is meant to work. Hope
this is useful...

Eric