InstaDMG 1.4b3 won’t use my base dmg

I've been successfully building images with InstaDMG 1.3. I decided to give 1.4b3 a run. On the same machine I copied over the disc image file of my base OS installer into the BaseOS folder and then moved all my other packages over to the CustomPKG folder for 1.4. When I run the 1.4 version it mounts the disc image for the Base OS install and then immediately gives me the following error: ERROR: the main install disk was not sucessfully mounted! It is mounted however. I know the files are good because they are working just fine under 1.3. Anyone have any thoughts or ideas why I'm getting this error with the new build? Thanks!