How do I get 1099-B losses to offset a Traditional->Roth conversion in a 1099-R?

Example: say the 1099-B has -15K in losses and 1099-R has 25K in "income" from the conversion. TurboTax seems to be treating entry of both forms as 25K in income, and not reducing net taxable income to 10K. Both forms are from the same Brokerage house.