Manually changing the taxable amount on form 1099-R to reduce tax liability

I received a 1099-R  for an annuity contract.  The annuity premium was funded from a Traditional IRA that I funded with after-tax dollars.  

 

The insurance company only knows that the annuity was funded from a qualified IRA account.  From their viewpoint, the entire amount of the annual annuity payment is taxable.  Based on my documentation, my annuity premium included 15 percent of after-tax dollars.

 

If I manually enter 15% of the Taxable amount in Box 5, TurboTax will not reduce the taxable amount of the annual annuity payment.  In addition, if I manually reduce the Taxable Amount by 15% in Box 2a, my tax  liability in TurboTax does not change.  BTW - Box 2b is checked - "Taxable amount not determined".

 

I would like to avoid paying double taxes on the 15% portion of my annuity payment that was funded with after tax dollars.  Any suggestions or ideas are welcome.  Thank you.