diff --git a/pkg/keystore/keystore.go b/pkg/keystore/keystore.go
index <HASH>..<HASH> 100644
--- a/pkg/keystore/keystore.go
+++ b/pkg/keystore/keystore.go
@@ -90,7 +90,7 @@ func checkSignature(ks *Keystore, prefix string, signed, signature io.Reader) (*
 	entities, err := openpgp.CheckArmoredDetachedSignature(keyring, signed, signature)
 	if err == io.EOF {
 		// otherwise, the client failure is just "EOF", which is not helpful
-		return nil, fmt.Errorf("keystore: no signatures found")
+		return nil, fmt.Errorf("keystore: no valid signatures found in signature file")
 	}
 	return entities, err
 }
keystore: improve missing signature error message

When a supplied signature file (e.g. .asc) contains no valid signature,
the current error message is a bit misleading; tweak it to try make it
clearer.