There are some situations where your cost basis would be zero, even if you had paid in. A typical example is when a stock or fund is liquidated, and the proceeds are sent as a "nondividend distribution". In this case, the basis is zero, and the proceeds to you are the excess over your cost basis.
If you think the cost is wrong. Contact the broker, and ask if there's a mistake, and get an explanation.
If you still think it's wrong, make absolutely sure you are correct. Then you can correct the cost by putting an adjustment in columns f/g of Form 8949, with the code "B". While it's unlikely that the IRS will question it, you should be prepared to prove that the adjustment is correct and the 1099-B is wrong.
