deferred tax issue of share-based payment expense

Dear sir, I'm having trouble understanding the calculation formular for the deferred tax asset temporary difference of share-based payment expense. In the BPP text book: The deferred tax asset temporary difference is measured as: Carrying amount of share-based payment expense 0 Less: tax base of share-based payment expense (estimated amount tax authorities will permit as a deduction in future periods, based on year end information) (X) Temporary difference (X) Deferred tax asset at X% X Could you please kindly clarify why the carrying amount of share-based payment expense is always zero? I think since the the company recognises an expense for the employee services received in return for the share options granted, the carrying amount of share-based payment expense should be X. And the related tax deduction does not arise until the share options are exercised, the tax base of should be zero. Could you kindly point out what's wrong of my thinking? Thanks.