1099-B PROCEEDS FROM BROKER AND BARTER EXCHANGE TRANSACTIONS

Hello,

My employer gives an option to sell a portion of RSUs to cover for taxes. The broker has sent me this form 

1099-B PROCEEDS FROM BROKER AND BARTER EXCHANGE TRANSACTIONS with

Long Term - Noncovered Securities

Cost basis is given as NOT ON FILE since the broker sells my existing stocks to cover for taxes.

 

What cost basis should i use