18 Pa.C.S.A. §304.  Ignorance or mistake.

Ignorance or mistake as to a matter of fact, for which there is reasonable explanation or excuse, is a defense if:

(1)  the ignorance or mistake negatives the intent, knowledge, belief, recklessness, or negligence required to establish a material element of the offense; or

(2)  the law provides that the state of mind established by such ignorance or mistake constitutes a defense.