One implementation details about NWK_AUTO_POLL_EVT in Z stack

When I check the code about the NWK_AUTO_POLL_EVT in Z stack(nwk.c), I found a strange statement I don’t understand. There is an if statement

If(NwkPollReq( 0 ) != ZMacSuccess)

// Start the timer to try again later

            nwk_set_poll_evt( zgPollRate );

However, this dose not make sense to me since this NwkPollReq( 0 ) always return 0 by checking the code

So I don’t understand if I am missing something?

Thanks

Rui