The court sustained an unsecured creditor's objection to the debtor's plan because a class of allegedly impaired creditors who voted to accept the plan had not filed a proof of claim, so their claim had not "been allowed under § 502" as required to vote on the plan by § 1126(a). Because this issue was dispositive, the court did not reach the parties' argument concerning the applicability of the absolute priority rule to an individual debtor.