Does your taxpayer have any money in any other Trad IRA account, or this Trad IRA account that received the nondeductible contribution has any other non-basis funds? If everything looks right and there is tax, it would be common for Pro Rata tax, because someone didn't understand how backdoor will be computed. It's essentially an immediate conversion, but if there are other Trad IRA account(s), other amounts that were not yet taxed (such as earnings), then this conversion will be taxed pro rata.

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