(a) General rule.--It is a defense that the actor engaged in the conduct charged to constitute an offense because he was coerced to do so by the use of, or a threat to use, unlawful force against his person or the person of another, which a person of reasonable firmness in his situation would have been unable to resist.
(b) Exception.--The defense provided by subsection (a) of this section is unavailable if the actor recklessly placed himself in a situation in which it was probable that he would be subjected to duress. The defense is also unavailable if he was negligent in placing himself in such a situation, whenever negligence suffices to establish culpability for the offense charged.