Does a district court's denial of a proposed jury instruction should be reviewed for abuse or de novo?

Our circuit has not resolved the issue of whether a district court's denial of a proposed jury instruction should be reviewed for abuse of discretion or de novo. United States v. Streit, 962 F.2d 894, 897 (9th Cir.), cert. denied, 113 S.Ct. 431 (1992). However, when the issue raised is whether a defendant's theory of the case is adequately covered by other instructions, that presents a question of law which we review de novo. United States v. Gomez-Osorio, 957 F.2d 636, 642 (9th Cir.1992).