It is well settled that tbe plea of payment is an affirmative one, and tbe general rule is that tbe burden of showing payment must be assumed by tbe party interposing it. Davis v. Dockery, 209 N.C. 272, 183 S.E. 396; Furst v. Taylor, 204 N.C. 603, 169 S.E. 185; Collins v. *792 Vandiford, 196 N.C. 237, 145 S.E. 235; Swan v. Carawan, 168 N.C. 472, 84 S.E. 699. See also Joyce v. Sell, 233 N.C. 585, 64 S.E. 2d 837; 8 Am. Jur., Bills and Notes, Sec. 1035; 40 Am. Jur., Payment, Sec. 278. The burden of proof is a substantial right. Davis v. Dockery, supra; Collins v. Vandiford, supra.

Here tbe defendant’s plea of payment cast on him tbe burden of proving tbe affirmative of tbe issue thus raised. However, tbe trial court inadvertently placed tbe burden of proof on tbe plaintiff. This entitles tbe plaintiff to a

New trial.