Charles Johnson appeals the sentence the district court 1 imposed after he pleaded guilty to a drug charge. Following our review of counsel’s brief filed under Anders v. California, 386 U.S. 738, 87 S.Ct. 1396, 18 L.Ed.2d 493 (1967), as well as Johnson’s pro se supplemental brief, we conclude that Johnson cannot challenge an offense-level enhancement for possession of a dangerous weapon after stipulating to such an enhancement in the plea agreement, see United States v. Nguyen, 46 F.3d 781, 783 (8th Cir. 1995), and that his ineffective-assistance arguments would be more properly raised in a collateral proceeding, see United States v. Cain, 134 F.3d 1345, 1352 (8th Cir. 1998). In addition, we have reviewed the record independently for nonfrivolous issues under Penson v. Ohio, 488 U.S. 75, 109 S.Ct. 346, 102 L.Ed.2d 300 (1988), and we find none.

Accordingly, we affirm. Counsel’s motion to withdraw is granted.