RYAN, J., delivered the opinion of the court.
GILMAN, J. (pp. 523 - 524), delivered a separate concurring opinion.
COOK, J., concurred in both the opinion of the court and the separate concurrence.
OPINION
Chalmers Brown appeals the sentence he received upon his conviction for being a felon in possession of a firearm under 18 U.S.C. § 922(g) and for being an Armed Career Criminal under 18 U.S.C. § 924(e). He was initially sentenced to 360 months in prison, but on appeal that sentence was vacated and his case was remanded for new sentencing. United States v. Brown, 371 F.3d 854, 861 (6th Cir.2004). On remand Brown was sentenced to 280 months’ imprisonment, which he now appeals on the grounds that his sentence is invalid under United States v. Booker, 543 U.S. 220, 125 S.Ct. 738, 160 L.Ed.2d 621 (2005).
We acknowledge that because the district court believed, when sentencing Brown, that the United States Sentencing Guidelines were mandatory, and because, when Booker was decided, Brown’s case was in the appellate pipeline, his sentence violated Booker. But after careful consideration, we conclude that the district court’s “error” in following the law applicable on the day of Brown’s resentencing was harmless, because the sentence imposed was based upon a significant upward departure that resulted in a reasonable sentence. Therefore, we will AFFIRM Brown’s sentence.
I.
Brown was arrested in December 2001 for being a felon in possession of a firearm, in violation of 18 U.S.C. § 922(g). He pled guilty to this offense and at sentencing the district court imposed a sentence of confinement for 360 months under the 2001 Sentencing Guidelines. Brown, 371 F.3d at 857. Brown appealed, and a panel of this court vacated the sentence because it was unsure whether the upward departure was a product of the district court’s “inde*521pendent judgment.” Id. The district court was directed to resentence Brown, giving “independent thought to whether th[e] range was appropriate under the individual circumstances of Mr. Brown’s case.” Id. at 861.
A second sentencing hearing was held on August 26, 2004, which was before Booker was decided. The district court determined that the statutory range was 180 months to life, based upon Brown’s felon in possession of a firearm conviction, to which he had pled guilty, and the court’s conclusion that Brown was subject to the Armed Career Criminal Act of 1984, 18 U.S.C. § 924(e). The act applies where the defendant has three separate prior convictions for a “violent felony” or a “serious drug offense.” 18 U.S.C. § 924(e)(1) (West Supp.2005).
Although, as we have said, Booker had not been decided, Brown’s alert counsel, April Goode of the Tennessee bar, objected to the use of the Guidelines, citing Blakely v. Washington, 542 U.S. 296, 124 S.Ct. 2531, 159 L.Ed.2d 403 (2004), which Goode argued should apply to federal guideline sentencing. But the district court correctly observed that it was bound to follow the precedent of this circuit, which at the time was that the federal sentencing Guidelines were applicable, constitutional, and mandatory. United States v. Koch, 383 F.3d 436 (6th Cir.2004), vacated, 544 U.S. 995, 125 S.Ct. 1944, 161 L.Ed.2d 764 (2005).
In calculating an appropriate sentence under the guidelines, the district court began with the applicable punishment range for Brown’s felon in possession of a firearm conviction, then took into account, as relevant conduct, Brown’s involvement in an armed robbery committed three weeks before his arrest for the firearm offense. The court noted that Brown had discharged a firearm during the robbery, albeit a different weapon than the one for possession of which he was being sentenced. The court also observed that one of the robbery victims suffered serious bodily injury in the robbery. The result of including these “enhancing factors,” which were partially offset by a deduction for acceptance of responsibility, was a base offense level of 30.
The court then turned to Brown’s criminal history. After noting that 13 criminal history points were sufficient to place a defendant in the highest criminal history category, the court found that Brown had earned 53 points. The court, therefore, decided that an upward departure was appropriate. Relying on “the authority and the discretion” granted under U.S.S.G. § 4A1.3, the court departed upward from Offense Level 30 Category VI (168-210 months) to Offense Level 34 Category VI (262-327 months) and sentenced Brown to confinement for 280 months.
II.
Brown’s case was on direct appeal to this court when Booker was decided. Therefore, Brown may avail himself of the rulings announced in Booker. United States v. Oliver, 397 F.3d 369, 377 (6th Cir.2005). Because Brown raised his objection to the Guidelines before the district court, we employ de novo review. United States v. Jones, 399 F.3d 640, 649 (6th Cir.), cert. denied, — U.S.-, 126 S.Ct. 148, 163 L.Ed.2d 146 (2005). Additionally, if an error is found, Booker instructs “reviewing courts to apply ordinary prudential doctrines, determining, for example, whether the issue was raised below and [if it was not] whether it fails the ‘plain-error’ test.” Booker, 125 S.Ct. at 769. Because Brown did raise an objection to the Guidelines below, we do not conduct a plain-error analysis, but inquire instead whether the district court’s error — and we acknowledge that it erred in following this court’s *522precedent — was harmless. United States v. Christopher, 415 F.3d 590, 593 (6th Cir.2005); United States v. McDaniel, 398 F.3d 540, 546 (6th Cir.2005). In United States v. Hazelwood, 398 F.3d 792 (6th Cir.2005), this court held that a remand is necessary under the harmless error standard “unless we are certain that any ... error was harmless — ie. any such error ‘did not affect the district court’s selection of the sentence imposed.’ ” Id. at 801 (quoting Williams v. United States, 503 U.S. 193, 203, 112 S.Ct. 1112, 117 L.Ed.2d 341 (1992)).
III.
Brown first claims that the district court violated his Sixth Amendment rights under Booker because, in classifying him as an armed career offender under 18 U.S.C. § 924(e), the court made findings that at least three of Brown’s prior convictions were either for a “violent felony” or a “serious drug offense,” factual issues, Brown argues, that should have been decided by a jury. We reject this argument because this court has previously held that “Apprendi does not require the nature of prior convictions to be determined by a jury.” United States v. Barnett, 398 F.3d 516, 525 (6th Cir.2005).
We agree -with Brown, however, that the district court erred in sentencing him in the belief that the Guidelines were mandatory and, in calculating the appropriate sentencing range, made forbidden findings of fact. The court repeatedly stated that the Guidelines were mandatory. See id. The court also erred when it determined that Brown’s involvement in an armed robbery some three weeks prior to his arrest for the firearm offense was conduct relevant to the firearm offense to which he pled guilty in this case. We hold, however, that, as we explain below, under the harmless error standard of review that is appropriate in this case, we are “certain” these errors did not affect the district court’s selection of the sentence imposed.
IV.
It is apparent that Brown suffered no harm either from the court’s mistaken belief that the Guidelines were mandatory or its “relevant conduct” factfinding. It is true the district court believed the Guidelines to be mandatory and improperly enhanced Brown’s basic offense level after finding the events of the robbery relevant to the firearm offense. Nevertheless, any possible harm from these errors was nullified when the court later exercised its discretion to grant a substantial upward departure that increased the sentence considerably above the Guideline recommended range. Stated differently, it is impossible that the court’s Booker error added so much as one day to Brown’s sentence beyond what the discretionary upward departure added. Nor does this upward departure itself raise Booker problems since it was based on Brown’s criminal history and, in any event, was a discretionary decision by the district court. See Booker, 125 S.Ct. at 750.
For these reasons we are “certain” that the district court’s belief that the Guidelines were mandatory and its impermissible factfinding as to Brown’s relevant conduct “did not affect the district court’s selection of the sentence imposed,” Williams, 503 U.S. at 203, 112 S.Ct. 1112, which was based not upon a supposed mandatory Guideline range, but upon the court’s discretion to depart upward to reach a more severe sentence. The court’s sentencing errors were therefore harmless.
V.
Lastly, even though Brown offers no argument as to how the court *523might have acted unreasonably in granting an upward departure, we review Brown’s sentence for reasonableness under Booker. Christopher, 415 F.3d at 594. A sentence may be unreasonable when the court does not consider the applicable Guideline range or ignores the factors listed in 18 U.S.C. § 3553(a). Id. Here, the court considered the applicable Guideline range before departing upward and, in addition, mentioned “deterrence, incapacitation, just punishment, and rehabilitation” — all factors contained in § 3553(a), as the reasons for the sentence imposed. We conclude that Brown’s sentence is reasonable.
VI.
For the foregoing reasons, we AFFIRM the sentence imposed by the district court.