Marcus Roberts pleaded guilty to possession of a prohibited object by an inmate. In reliance on United States v. Booker, — U.S. -, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), he argues for the first time on appeal that the district court erred in imposing a sentence based on facts not alleged in the indictment, not admitted by him in court, and not proved to a jury beyond a reasonable doubt. Specifically, Roberts challenges the district court’s imposition of a two-level increase in his base offense level pursuant to U.S.S.G. § 2Dl.l(b)(3) on the ground that the object of the offense was the distribution of a controlled substance while in prison. He also argues that the district court erred in sentencing him pursuant to a mandatory application of the Sentencing Guidelines.

To demonstrate plain error, Roberts has the burden of showing an error that is obvious and that affects his substantial rights. United States v. Mares, 402 F.3d 511, 520 (5th Cir. 2005), petition for cert. filed (Mar. 31, 2005) (No. 04-9517). To show that the error affected his substantial rights, Roberts must show that “the sentencing judge — sentencing under an advisory scheme rather than a mandatory one — would have reached a significantly different result.” Id. at 521; see also United States v. Valenzuela-Quevedo, 407 F.3d 728, 732-34 (5th Cir. 2005). Roberts *753 has not shown that the district court would have imposed a different sentence. Accordingly, we AFFIRM the judgment of the district court.