Frank Robinson appeals the 84-month prison sentence the district court imposed after he pleaded guilty to being a felon in possession of a firearm. Robinson preserved at sentencing the question of the constitutionality of the federal Sentencing Guidelines after Blakely v. Washington, 542 U.S. 296, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004). For purposes of our analysis we assume that this case does not raise a Sixth Amendment issue, and we thus review for harmless error. See United States v. Booker,—U.S.-, 125 S.Ct. 738, 769, 160 L.Ed.2d 621 (2005). It is clear from the sentencing transcript that the district court made a conscious decision to sentence Robinson at the bottom of the Guidelines range, commenting that it felt even this sentence was too harsh. Given this record, we are left with “grave doubt” that the error of sentencing Robinson under a mandatory Guidelines scheme was harmless. See United States v. Haidley, 400 F.3d 642, 645 (8th Cir. 2005). Accordingly, we remand for resentencing.