I believe that you report the cost basis as zero, not negative. As somebody else responded, once your ROC distributions exceeded your cost, you should have been reporting those distributions as capital gains all along. I'm not sure if they are considered short or long-term capital gains though, so definitely check into that. It's the same net result conceptually, if you reported a cost basis of negative $10, that's just like saying you made an extra $10 in capital gains when you sold the stock.
