Empty Output

Periodically I run the instadmg.bash script and wind up with no image, there is a handy Create a read-only image hdiutil: couldn't unmount "disk3" - error 49153 hdiutil: convert failed - File exists pair of error messages before if says it finishes, i have rebooted and been ok after, i have also never gotten it to work on that particular OS ever again until i rebuild the system. Today first build failed, rebooted, second build failed, so I just tried cleaning out /tmp/instaDMG* and starting the process again, but if i could get a fix in case it fails for a 3rd time today it would be great. thanks.