APK: Invalid tar files

[td94]

Hello,
I'm trying to build pretty simple packages containing 2 small files and a simple post-install script.
This is working fine on deb and rpm packages, but I get problems with apk files.

The resulting apk seems to be correct and, interestingly, can also be installed directly from file. However, when I try to host them on an Artifactory alpine repository, the same file cannot be installed (Error: BAD Signature)

Digging further into it and comparing it to "official" apk packages, it looks like fpm produces somehow invalid tar files. From my understanding, apk files are just gzipped tarballs and can be extracted via tar -xzf .apk - at least, this can be done on the official packages. When trying to extract an apk file built with fpm, however, I get an error tar: invalid tar header checksum.