What is the test for the affirmative defense in a civil case?

In the case of an affirmative defense, however, the potential for misinterpretation is too great to permit ambiguity. An affirmative defense admits the defendant committed the acts charged, but seeks to establish a justification or excuse. In the absence of clear instructions, it is not unlikely that the jury would infer that the government has borne its burden and that it is up to the defendant to establish his justification.

Id.; see also United States v. Jackson, 726 F.2d 1466, 1469 (9th Cir.1984).