Payment in Advance Lease accounting

Hi Sir, I am a bit confused on the given example in your lecture on lease accounting treatment (Example 4, lessee accounting, page 83, Plum). The year 1 lease payment is paid in advance: $5000 on 1/1/2015. So the lease liability ($22730) is the present value of the future cash payments in 4 instalments: $5000 each on 1/1 from 2016 to 2019. I don't get it why the $5000 needs to be deducted from the lease liability to calculate the amortised finance cost for year 1. I assume that $5000 is the initial payment on 1/1/2015, right? Shouldn't that not part of the liability rationaled in the above paragraph? I concur in year 2, $5000 should be deducted from the liability brought forward from year 1.