Aliak Thor appeals the 60-month prison sentence the district court 1 imposed after he pleaded guilty to conspiring to engage in the sexual trafficking of a minor in violation of 18 U.S.C. §§ 371, 1591. Mr. Thor’s counsel has moved to withdraw and has filed a brief under Anders v. California, 386 U.S. 738, 87 S.Ct. 1396, 18 L.Ed.2d 493 (1967), arguing that Mr. Thor was denied the effective assistance of counsel, and that the sentence imposed was harsher than anticipated.

The arguments fail. Any ineffective-assistance claim should be raised in 28 U.S.C. § 2255 proceedings, see United States v. Hughes, 330 F.3d 1068, 1069 (8th Cir. 2003), and Mr. Thor’s plea-agreement stipulation to the specific sentence that was actually imposed forecloses any argument relating to that sentence, see United States v. Nguyen, 46 F.3d 781, 783 (8th Cir. 1995) (defendant who explicitly and voluntarily exposes himself to specific sentence may not challenge that punishment on appeal).

Having reviewed the record independently under Penson v. Ohio, 488 U.S. 75, 109 S.Ct. 346, 102 L.Ed.2d 300 (1988), we find no nonfrivolous issues. Accordingly, *576 we grant counsel’s motion to withdraw, and we affirm.