then the debt shall, to the extent charged off during the taxable year, be conclusively presumed to have become worthless, or worthless only in part, as the case may be, during such taxable year. But no such debt shall be so conclusively presumed to be worthless, or worthless only in part, as the case may be, if the amount so charged off is not claimed as a deduction by the taxpayer at the time of filing the return for the taxable year in which the charge-off takes place.
26 C.F.R. §1.166-2