no valid user IDs after changing key expiration time

[flapflap]

Hi list,

I have a key for some years which did not have an expiration time.  Now,
I finally want to change this and changed the expiration time (via
--edit-key then expire, save).

To test if everything is now working as it should, I exported the keys
(--export, --export-secret-keys), deleted the key from my keyring
(--delete-secret-keys, --delete-keys).

Afterwards, I imported my public key and the output is fine.
But when I import the secret key gpg says:

gpg: key $MYKEY: secret key imported
gpg: key $MYKEY: no valid user IDs
gpg: this may be caused by a missing self-signature
gpg: Total number processed: 1
gpg:           w/o user IDs: 1
gpg:       secret keys read: 1
gpg:   secret keys imported: 1

So, it says "no valid user IDs".

However, --list-keys and --list-secret-keys both show my UIDs.

Should I be worried by the warning or is this normal behaviour?

(GnuPG version is 2.0.25 and libgcrypt is 1.5.0)

Thanks in advance,
~flapflap