What is the test for giving a supplemental jury instruction at the close of evidence?

While the rule applies to instructions requested by a party before the close of evidence, it does not apply "in the absence of a requested instruction which the court initially declined to give ... if a supplemental jury instruction given in response to a jury's question introduces a new theory to the case, the parties should be given an opportunity to argue the new theory ... to prevent unfair prejudice." United States v. Fontenot, 14 F.3d 1364, 1368 (9th Cir.), cert. denied, --- U.S. ----, 115 S.Ct. 431, 130 L.Ed.2d 343 (1994).