Unless displaced by the provisions of this chapter, the principles of law and equity, including, but not limited to, the law relating to capacity to contract, mutuality of obligations, principal and agent, real property, public health, safety and fire prevention, estoppel, fraud, misrepresentation, duress, coercion, mistake, bankruptcy, or other validating or invalidating cause supplement its provisions.
Ala. Code § 35-9A-103 (1975)