That means I applied the wall to a non-periodic boundary, but came up with the following error message (copied and paste directly):
ERROR: Cannot use fix wall in periodic zimension
This does not occur in the lammps-17Nov09 version, but did occur in the latest lammps-21Nov09 version.
The 19 Nov 2009 patch description is:
Reformulated the fix wall/lj93, fix wall/lj126, and fix wall/colloid commands to be self-consistent. Each command can now be used to specify up to 6 flat walls, be used with lattice or box units, and have a velocity or oscillatory motion applied to the walls.