Please help!

2 partners in an LLC (Partnership) in 2020.  During year, Partner A buys out Partner B.  At end of year, Partner A has negative $100K in capital account.  Partner B has $0 obviously.

Day 1 of 2021, Partner A makes an S-election.  How should the negative $100K be treated?  

Fwiw: 2021 net income was $215,000.

Really stumped by this!