starting with the regs 1.401(k)-1(a)(6)(iii) Timing of self-employed individual’s cash or deferred election. For purposes of paragraph (a)(3)(iv) of this section, a partner’s compensation is deemed currently available on the last day of the partnership taxable year and a sole proprietor’s compensation is deemed currently available on the last day of the individual’s taxable year. Accordingly, a self-employed individual may not make a cash or deferred election with respect to compensation for a partnership or sole proprietorship taxable year after the last day of that year. See §1.401(k)-2(a)(4)(ii) for the rules regarding when these contributions are treated as allocated. ............ of course, in the case of a partner, the compensation hasn't necessarily been determined until after the end of the year, but the election itself must be made before the end of the year .......... the example used by Corbel is (which sounds almost exactly like your scenario) What is the timing for a participant to make a deferral election? A participant must make an election to defer his/her compensation to a 401(k) plan before the compensation is available. Treas. Reg. §1.401(k)-1(a)(3). Example. Ellen, a shareholder-employee, after the close of the plan year, determines that she could have deferred more to the 401(k) plan. Accordingly, she writes a check to the trustee for an additional $4,000 and asks the trustee to treat is as an elective deferral. The contribution is not an elective deferral because the election was made after the compensation was already available to the participant. http://www.relius.net/News/TechnicalUpdateDetails.aspx?T=P&1=1&ID=1012
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sorry, that is the best I can provide, if that is not enough I can't help you