26 U.S.C. § 4974 : US Code - Section 4974: Excise tax on certain accumulations in qualified retirement plans

    (a) General rule
      If the amount distributed during the taxable year of the payee
    under any qualified retirement plan or any eligible deferred
    compensation plan (as defined in section 457(b)) is less than the
    minimum required distribution for such taxable year, there is
    hereby imposed a tax equal to 50 percent of the amount by which
    such minimum required distribution exceeds the actual amount
    distributed during the taxable year. The tax imposed by this
    section shall be paid by the payee.
    (b) Minimum required distribution
      For purposes of this section, the term "minimum required
    distribution" means the minimum amount required to be distributed
    during a taxable year under section 401(a)(9), 403(b)(10),
    408(a)(6), 408(b)(3), or 457(d)(2), as the case may be, as
    determined under regulations prescribed by the Secretary.
    (c) Qualified retirement plan
      For purposes of this section, the term "qualified retirement
    plan" means - 
        (1) a plan described in section 401(a) which includes a trust
      exempt from tax under section 501(a),
        (2) an annuity plan described in section 403(a),
        (3) an annuity contract described in section 403(b),
        (4) an individual retirement account described in section
      408(a), or
        (5) an individual retirement annuity described in section
      408(b).

    Such term includes any plan, contract, account, or annuity which,
    at any time, has been determined by the Secretary to be such a
    plan, contract, account, or annuity.
    (d) Waiver of tax in certain cases
      If the taxpayer establishes to the satisfaction of the Secretary
    that - 
        (1) the shortfall described in subsection (a) in the amount
      distributed during any taxable year was due to reasonable error,
      and
        (2) reasonable steps are being taken to remedy the shortfall,

    the Secretary may waive the tax imposed by subsection (a) for the
    taxable year.