Can a prosecutor improperly vouched for the credibility of a government witness by stating that truthfulness was a condition of a plea agreement?

In United States v. Roberts, 618 F.2d 530, 532 (9th Cir.1980), this court held that a prosecutor had improperly vouched for the credibility of a government witness by stating that truthfulness was a condition of the witness' plea agreement and that a government agent was in the audience monitoring the witness' testimony. There we warned that