Anthony Young appeals his 15-month sentence for marriage fraud. Citing United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), he argues that the district court erred in increasing his criminal history based on its conclusion that, at the time of his offense, he had been released from custody for less than two years and was on parole. Young also argues that the district court committed Fanfan error when it sentenced him pursuant to a mandatory guidelines system. Because we conclude that the district court-committed Fanfan error when it sentenced Young pursuant to a mandatory .guidelines system, see United States v. Valenzuela-Quevedo, 407 F.3d 728, 733 (5th Cir.) cert. denied, — U.S. -, 126 S.Ct. 267, 163 L.Ed.2d 240 (2005), we decline to address Young’s argument that the district court’s factfinding amounted to Booker error. See United States v. Akpan, 407 F.3d 360, 377 n. 62 (5th Cir. 2005).

We review a preserved Fanfan challenge for harmless error. United States v. Rodriguez-Mesa, 443 F.3d 397, 404 (5th Cir. 2006). The Government has not met its arduous burden of demonstrating that the district court would have imposed the same sentence absent its mandatory application of the Sentencing Guidelines. See United States v. Zamora-Vallejo, 470 F.3d 592, 595 (5th Cir. 2006); United States v. Garza, 429 F.3d 165, 170 (5th Cir. 2005), cert. denied, — U.S. -, 126 S.Ct. 1444, 164 L.Ed.2d 143 (2006). Accordingly, we VACATE Young’s sentence and REMAND for resentencing.