The "path" argument must be of type string

Hi,

I'm trying to create a solo but the grunt execution is not successful because of the warning:
`Processing readkit.epub/testepub/OEBPS/content.opf
Warning: The "path" argument must be of type string. Received type object Use --force to continue.`

However, if I use -f, grunt finishes completely but doesn't create the solo directory.

Regards

