Could not extend expiration date

[Pericle Unico]

I wasn't able to extend the validity of my gpg key, because instead of
promting me with the passphase request I got this error:


but I've a private key, and I'm able both to sign and decrypt.

Anyone can help?

Below details about the error

### Version Info: SO is Windows 7 ###

set LANG=en

C:\Users\me>gpg --help
gpg (GnuPG) 2.0.22 (Gpg4win 2.2.1)
libgcrypt 1.5.3
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: C:\Users\me\AppData\Roaming\gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ?, ?
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

### Signing a document is ok ###

C:\Users\me\Downloads>gpg -vvv -s volterra_saggi_scientifici.epub
gpg: using character set `CP850'
gpg: using PGP trust model
gpg: key 3D2B665D: accepted as trusted key
gpg: using subkey 658682A5 instead of primary key 3D2B665D

You need a passphrase to unlock the secret key for
user: "MyName <repl at ced>"
gpg: using subkey 658682A5 instead of primary key 3D2B665D
2048-bit DSA key, ID 658682A5, created 2012-02-26 (main key ID 3D2B665D)

File `volterra_saggi_scientifici.epub.gpg' exists. Overwrite? (y/N) y
gpg: writing to `volterra_saggi_scientifici.epub.gpg'
gpg: DSA/SHA256 signature from: "658682A5 MyName <repl at ced>"


### Editing to extend expiration date fails ###

C:\Users\me\Downloads>gpg -vvv --edit-key repl at ced
gpg (GnuPG) 2.0.22; Copyright (C) 2013 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

gpg: using character set `CP850'
gpg: using PGP trust model
gpg: key 3D2B665D: accepted as trusted key
Secret key is available.

pub 2048D/3D2B665D created: 2012-02-16 expires: 2014-02-16 usage: SC
trust: ultimate validity: ultimate
sub 3072g/791B80B4 created: 2012-02-16 expires: 2014-02-16 usage: E
sub 2048D/658682A5 created: 2012-02-26 expires: 2014-02-25 usage: S
[ultimate] (1). MyName <repl at ced>

gpg> expire
Changing expiration time for the primary key.
Please specify how long the key should be valid.
0 = key does not expire
<n> = key expires in n days
<n>w = key expires in n weeks
<n>m = key expires in n months
<n>y = key expires in n years
Key is valid for? (0) 2y
Key expires at 02/07/16 20:28:52 W. Europe Standard Time
Is this correct? (y/N) y
gpg: secret key parts are not available
gpg: make_keysig_packet failed: Unusable secret key


thanks