What is the test for commenting on a defendant's exercise of the Fifth Amendment?

The government is not permitted to comment on a defendant's exercise of his right not to testify. See Griffin v. California, 380 U.S. 609, 615 (1965). The government may, however, "comment on defendant's failure to call witnesses to contradict the factual character of the Government's case, as well as on defendant's failure to support his own factual theories with witnesses." United States v. McDermott, 918 F.2d 319, 327 (2d Cir. 1990). We have explained that such comments violate the Fifth Amendment only "when

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