What is the test for a defendant's claim that the Government witnesses are lying?

We have held that a defendant is "entitled to have instructions presented relating to any theory of defense for which there is any foundation in the evidence, no matter how weak or incredible . . . ." United States v. Platt, 435 F.2d 789, 792 (2d Cir. 1970), quoting United States v. O'Connor, 237 F.2d 466, 474 n. 8 (2d Cir. 1956). This language is broad enough to include the theory that the Government's witnesses are lying. United States v. Vole, 435 F.2d 774 (7th Cir. 1970). While the Government accepts this formulation, it argues that there was in this case no "foundation in the evidence" for the theory presented by defendant.