My 1099-B is missing cost basis information. How do I enter it myself such that TurboTax won't show "Needs Review"?

The item is a noncovered stock sale (not reported to the IRS by the broker). The broker has no cost basis information, so I need to enter it myself. When I do, no matter what I enter into TurboTax the item still shows "Needs Review" with no details about what exactly triggers that warning.