I found the following error from gpgv to be a little opaque:

gpgv: unknown type of key resource 'trustedkeys.kbx'
gpgv: keyblock resource '/home/lars/.gnupg/trustedkeys.kbx': General error
gpgv: Can't check signature: No public key

It turns out that’s gpg-speak for “your trustedkeys.kbx keyring doesn’t exist”. That took longer to figure out than I care to admit. To get a key from your regular public keyring into your trusted keyring, you can run something like the following: