detached signature, "can't hash datafile: No data"

[S.B.]

Hello,

I wanted to verify an install file so I downloaded file.dmg and the
accompanying detached signature.asc.  The public key was imported and
verified.  Using GnuPG, I used the command:
gpg --verify signature.asc file.dmg

and..

"Good signature from..."

However, when I try to verify signature.asc independently using the command:
gpg --verify signature.asc

it states:
gpg: no signed data
gpg: can't hash datafile: No data

Shouldn't I be able to verify the signature independently?

S.B.