ORDER
After William King pleaded guilty to distributing crack cocaine in violation of 21 U.S.C. § 841(a)(1), and was sentenced to 18 months’ imprisonment, he filed a notice of appeal. His appointed counsel, however, seeks to withdraw pursuant to Anders v. California, 386 U.S. 738, 87 S.Ct. 1396, 18 L.Ed.2d 493 (1967), because he believes all potential appellate issues are frivolous. Counsel’s supporting brief is facially adequate and King has not responded, see Circuit Rule 51(b), so we limit our review to the potential issues counsel identifies. See United States v. Tabb, 125 F.3d 583, 584 (7th Cir.1997) (per curiam); United States v. Wagner, 103 F.3d 551, 553 (7th Cir.1996). We agree with counsel that all identified issues would be frivolous, and therefore grant counsel’s motion to withdraw and dismiss the appeal.
Counsel next considers whether King could challenge his prison sentence. First, counsel evaluates whether King could argue that the district court misapplied the guidelines. King told the district court that he had no objections to the court’s guidelines calculations, effectively waiving his right to appeal those calculations. United States v. Staples, 202 F.3d 992, 995 (7th Cir.2000). As waiver extinguishes any error and precludes appellate review, id., counsel is correct that an appeal of the calculations would be frivolous. Second, counsel considers and rejects an appeal on the ground that the district court should have reduced King’s sentence by more than the six months it did for substantial assistance under U.S.S.G § 5K1.1. We agree with counsel that because we lack jurisdiction to review such claims, see United States v. Newman, 148 F.3d 871, 875 n. 2 (7th Cir.1998), an appeal on this ground would also be frivolous. Last, counsel contemplates a constitutional challenge to King’s sentence based on the sentencing disparity between crack and powder cocaine. But as counsel notes, this circuit has consistently rejected such challenges. See, e.g., United States v. Westbrook, 125 F.3d 996, 1010 n. 16 (7th Cir.1997) (collecting cases). We agree with counsel that sheer repetition is insufficient to overcome the substantial precedent making an argument on this ground frivolous.
Accordingly, we GRANT counsel’s motion to withdraw and DISMISS this appeal.