There's a line in pstopnm which reads as follows:

set xres = `(echo "scale=4";echo "if($xres>$yres)$yres";echo
"if($yres>$xres)$xres")|bc -l`

It will fail if $xres equals $yres.  It should read:

set xres = `(echo "scale=4";echo "if($xres>$yres)$yres";echo
"if($yres>$xres)$xres";echo "if($xres==$yres)$xres")|bc -l`