cost basis for follow up payment

Years ago, I left a company where I had some stock options. The company was eventually acquired by another company, and I received a payout for my shares, and I filed the proceeds as capital gains. The following year, I received an additional check for ~$100. There was no explanation, but I assumed there was some sort of adjustment in the final price.

I have a 1099-B for this additional payment, but it has no cost basis. The Box 1A Qty Sold lists the total number of shares that I had, so any cost would have already been factored in to the total capital gain in the prior year. Is my cost basis just $0 for this?