Lorenzo J. Villasenor appeals 'the sentence imposed following his guilty-plea *749conviction for possession with the intent to distribute 66.25 kilograms of marijuana, in violation of 21 U.S.C. § 841(a)(1) and (b)(1)(C). Finding no error, we affirm.

Villasenor first argues that, in light of United States v. Booker, — U.S. -, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), the district court erred in imposing a sentence utilizing the Sentencing Guidelines as mandatory. As Villasenor did not preserve this issue, we review only for plain error. See United States v. Mares, 402 F.3d 511, 520 (5th Cir. 2005), petition for cert. filed (Mar. 31, 2005) (No. 04-9517); see also United States v. Malveaux, 411 F.3d 558 (5th Cir. 2005). Application of the Guidelines as mandatory, even absent a Sixth Amendment violation as is the case here, is plain or obvious error after Booker. See United States v. Valenzuela-Quevedo, 407 F.3d 728, 733-34 (5th Cir. 2005). However, Villasenor cannot show that the error affected his substantial rights because the record does not indicate that the district court would have imposed a lower sentence under an advisory, rather than a mandatory, Guidelines scheme. See id.

Villasenor’s second argument, that 21 U.S.C. § 841 is unconstitutional, is foreclosed by United States v. Slaughter, 238 F.3d 580 (5th Cir. 2000). Villasenor concedes this point, but raises it to preserve the matter for further review.

AFFIRMED.