signing failed with master key when I have stronger subkeys

[Dongsheng Song]

Hi,

Here is my keyring status after I removed the secret key of these
stronger subkeys:

    sec   rsa2048/46D397FF 2008-02-02
    ssb   rsa2048/7547A8A9 2008-02-02
    ssb#  brainpoolP512r1/DD1C5659 2015-06-24
    ssb#  brainpoolP512r1/24BEAC25 2015-06-24
    ssb#  rsa4096/F7BC1BF1 2015-06-24

Then I can not signing anymore even when I use --default-key or
--local-user to specify 46D397FF or 7547A8A9:

    gpg: signing failed: No secret key

After I delete these stronger subkeys completely:

    sec   rsa2048/46D397FF 2008-02-02
    ssb   rsa2048/7547A8A9 2008-02-02

Then signing action succeeded. Bugs, features, or misused ?

Regards,
Dongsheng