You would pay the most tax if your cost basis was zero, so you aren’t going to get in trouble for reporting a zero cost basis. If you claim anything else as the cost basis, you have the burden of proving if audited, and if you can’t prove it, the IRS is likely to set the cost basis to zero so that you pay maximum tax. Claiming zero now is a defensive strategy that can’t get you in trouble, which will result in paying more tax but taking less risk than guessing an approximate basis and waiting to see if you get audited.
