FCFF - WCinv and tax Rate


It must be simple, I should have learned it from CFAI but this question came to my mind while solving a DCC for a Merger in Corp Finance part of the study,

So, when I calculate the FCFF I subtract WCinv, CAPEX, Non Cash charges, changes in deferred tax and NET INTEREST AFTER TAX.

My question is, why only Interest after tax and not, for example, WCinv, given that it was included in REV and COGS, then taxed to arrive at the NI figure… why isn’t it WCin(1-tax)


I believe WCInv is not a part of the income statement and NI calculations