Is a federal judge's failure to depart downward from the sentencing guidelines reviewable on appeal?

Appellant challenges the district court's failure to depart downward from the sentencing guidelines. Although he acknowledges that a discretionary refusal to depart is not subject to review on appeal, he claims that this case is reviewable because the judge was erroneous in construing his authority to depart. Such cases are reviewable. United States v. Belden, 957 F.2d 671, 676 (9th Cir.1992). The interpretation of sentencing guidelines is reviewed de novo. United States v. Lawrence, 916 F.2d 553, 554 (9th Cir.1990).