Can a prosecutor comment upon a defendant's assertion of his Fifth Amendment right not to testify?

The test to judge impermissible comment upon a defendant's assertion of his Fifth Amendment right not to testify is whether the language used was manifestly intended or was of such a character that the jury would naturally and necessarily take it to be a comment on the failure to testify. United States v. Fleishman, 684 F.2d at 1343. A prosecutor may properly comment upon the defendant's failure to present exculpatory evidence, so long as it is not phrased to call attention to the defendant's own failure to testify. See United States v. Passaro, 624 F.2d 938, 944 (9th Cir.1980), cert. denied, 449 U.S. 1113, 101 S.Ct. 925, 66 L.Ed.2d 842 (1981).