Can a plaintiff raise the statute of limitations defense in an action for a refund from the IRS?

Presently, I.R.C. 6401(a) provides: "Assessment and collection after limitation period.--The term 'overpayment' includes that part of the amount of the payment of any internal revenue tax which is assessed or collected after the expiration of the period of limitation properly applicable thereto." We have held, based on 6401(a), that a taxpayer may raise the statute of limitations defense in an action for a refund. Walker v. IRS, 333 F.2d 768, 771 & n. 3 (9th Cir.1964). Thus, Sokolow fails the second prong of the test, because he does have an adequate remedy at law: to pay the tax and then challenge the validity of the tax liability in a refund suit. Therefore, Sokolow has not established that injunctive relief should be granted under the judicial exception to the Anti-Injunction Act.