Mender 2.0 beta / mender-artifact 3.0.0b1: "Error selecting images for modification: error validating signature"

Oki, I see it

$ mender-artifact modify my-update-1.0-signed.mender -k private.key
Error selecting images for modification: error validating signature
$ mender-artifact -version
mender-artifact version 3.0.0b1