26 C.F.R. § 20.2031-4
The fair market value of notes, secured or unsecured, is presumed to be the amount of unpaid principal, plus interest accrued to the date of death, unless the executor establishes that the value is lower or that the notes are worthless. However, items of interest shall be separately stated on the estate tax return. If not returned at face value, plus accrued interest, satisfactory evidence must be submitted that the note is worth less than the unpaid amount (because of the interest rate, date of maturity, or other cause), or that the note is uncollectible, either in whole or in part (by reason of the insolvency of the party or parties liable, or for other cause), and that any property pledged or mortgaged as security is insufficient to satisfy the obligation.
26 C.F.R. § 20.2031-4