Issue with embedding covers

Hello,

I just discovered this awesome tool and used it to merge a folder with about 10 mp3s into one m4b-file. It worked great.
The only thing that did not work was embedding a cover file. Whatever i tried, the tool always threw errors.

If i used a cover.png or cover.jpg it threw the following error:
could not convert cover.png to /Users/myname/Desktop/test-tmpfiles/1-finished.m4b

If i didnt put any file into the folder it showed:
extracting cover to cover.png/cover.jpg failed - maybe there was no cover embedded in cover.png

Is there any fix for this? Can provide more information if needed.

The only thing that worked was using --skip-cover, otherwise the conversion into one file wouldnt even finish.