What is the standard of review for a prosecutor's closing argument against a defendant's Fifth Amendment rights?

We review de novo any claims that the prosecutor's closing argument violated the defendant's Fifth Amendment rights. United States v. Mares, 940 F.2d 455, 461 (9th Cir.1991); but see United States v. Hoac, 990 F.2d 1099, 1103-04 (9th Cir.1993) (unclear whether standard of review de novo or abuse of discretion), petition for cert. filed, --- U.S.L.W. ---- (U.S. Nov. 3, 1993) (No. 93-6625).

We must determine whether the prosecutor's comments were such that the jury would naturally and necessarily take it to be a comment on the defendant's failure to testify. United States v. Castillo, 866 F.2d 1071, 1083 (9th Cir.1988) (prosecutor's comment on defense counsel's failure to call witness not violation of constitutional right). A single isolated comment will not be grounds for reversal when followed by a curative instruction. Lincoln v. Sunn, 807 F.2d 805, 809 (9th Cir.1987).