18 Pa.C.S.A. §309.  Duress.

(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.