Where the district court failed to advise Vonn of his right to counsel under rule 11(c)(3) of the Federal Rules of Criminal Procedure, the reviewing court ordered that his plea be withdrawn. The error was not harmless because nothing in the record, other than the fact that Vonn had counsel at the plea hearing, showed that Vonn was aware of his right to counsel at trial. While some unreported colloquy may have shown the plea to be harmless, it was not sufficient to satisfy the affirmative showing requirement.