1099-B - what to enter if cost basis date is n/a because it was a principal payment

[kmvt]

I have a 1099-B with an unknown cost basis... the date acquired says n/a and in the "additional info" it says it was  a principal payment (so not a sale).  I don't think this income is supposed to be taxed, but I don't know what to enter in turbo tax for the date and/or the cost basis. There are no "n/a" options!