Odd bug when reading raw gray image

I'm trying to read an image I've generated with this program:
http://www.worldserver.com/turk/opensource/#ZonePlate

It works fine at a 'low' resolution:
$ ./ZonePlate -w 3600 -h 3600 -s 16 -o z3600.gray
$ convert -size 3600x3600 z3600.gray PNG24:z3600.png

But at the next resolution up it throws errors:
$ ./ZonePlate -w 7200 -h 7200 -s 16 -o z7200.gray
$ convert -size 7200x7200 z7200.gray PNG24:z7200.png
Error: /undefined in ??N
Operand stack:

Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   -- 
nostringval--   2   %stopped_push   --nostringval--   -- 
nostringval--   --nostringval--   false   1   %stopped_push   1   3    
%oparray_pop   1   3   %oparray_pop   1   3   %oparray_pop   1   3    
%oparray_pop   .runexec2   --nostringval--   --nostringval--   -- 
nostringval--   2   %stopped_push   --nostringval--
Dictionary stack:
   --dict:1128/1686(ro)(G)--   --dict:0/20(G)--   --dict:71/200(L)--
Current allocation mode is local
Current file position is 18
AFPL Ghostscript 8.54: Unrecoverable error, exit code 1
Error: /undefined in ??N
Operand stack:

Execution stack:
   %interp_exit   .runexec2   --nostringval--   --nostringval--   -- 
nostringval--   2   %stopped_push   --nostringval--   -- 
nostringval--   --nostringval--   false   1   %stopped_push   1   3    
%oparray_pop   1   3   %oparray_pop   1   3   %oparray_pop   1   3    
%oparray_pop   .runexec2   --nostringval--   --nostringval--   -- 
nostringval--   2   %stopped_push   --nostringval--
Dictionary stack:
   --dict:1128/1686(ro)(G)--   --dict:0/20(G)--   --dict:71/200(L)--
Current allocation mode is local
Current file position is 18
AFPL Ghostscript 8.54: Unrecoverable error, exit code 1
convert: Postscript delegate failed `z7200.gray': No such file or  
directory.
convert: missing an image filename `PNG24:z7200.png'.


I've placed both input files (z3600.gray & z7200.gray) online here:
http://pricepages.org/magick/

I'm running Mac OS 10.5.2 and the latest ImageMagick 6.4.0.

Thanks,
Seth