Can a prosecutor refer to the guilty pleas of co-defendants as evidence of a defendant's guilt?

We held in United States v. Halbert that, although the references by the prosecutor during trial to the guilty pleas of co-defendants were not impermissible, the failure of the court to instruct the jury that the guilty pleas of co-defendants could not be considered as evidence of a defendant's guilt was reversible error. 640 F.2d 1000, 1006-07 (9th Cir.1981). However, in that case the evidence of the co-defendant's guilty pleas was offered over defense counsel's objections, and the curative instruction was necessary to dispel the danger of misuse by the jury. Id. at 1006.