Jose Rocha-Martinez appeals the sentence imposed following his guilty-plea conviction of transporting an illegal alien for commercial advantage or private financial gain. He argues that the district court committed reversible error under United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005), by sentencing him pursuant to a mandatory application of the guidelines. As the government concedes, Rocha preserved this issue for review by objecting in the district court based on Blakely v. Washington, 542 U.S. 296, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004) . See United States v. Walters, 418 F.3d 461, 462-63 (5th Cir. 2005). Accordingly, the question “is whether the government has met its burden to show harmless error beyond a reasonable doubt.” Id. at 464.

The district court erred by sentencing Rocha under the mistaken belief that the guidelines were mandatory. See United States v. Valenzuela-Quevedo, 407 F.3d 728, 732 (5th Cir.), cert. denied, — U.S. -, 126 S.Ct. 267, 163 L.Ed.2d 240 (2005) . The court sentenced Rocha in the middle of the guidelines range and did not state what sentence it would impose if the guidelines were held unconstitutional. In these circumstances, the government has not met its “arduous burden” of showing that the error was harmless. United States v. Garza, 429 F.3d 165, 170 (5th Cir. 2005) (internal quotation marks omitted). Accordingly, we vacate the sentence and remand for resentencing. We do not reach Rocha’s alternative argument that the district court committed a Sixth Amendment violation. See United States v. Akpan, 407 F.3d 360, 377 n. 62 (5th Cir. 2005).

SENTENCE VACATED; REMANDED FOR RESENTENCING.