zte asking for PUK NOT NETWORK CODE

I have one Grand X (Z787) from Cricket Wireless, that asks for PUKcode when any sim is inserted. Normally, puk is for sim cards that have been given wrong pin. But this one asks for puk even for simcards that are not pin locked. If it was asking for network code i would simply buy a code, but what does PUK code mean? Any suggestion would be appreciated. Thanks!!!