If the prior agreement to which the written instrument fails to conform was not expressed in writing, the agreement may be proved by the testimony of the parties or other persons who were present when it was made. Allowing this testimony regarding the parties' actual agreement is an exception to the parol evidence rule, which holds that parol statements or evidence are not admissible to vary or contradict the terms of a written instrument Spear v. MacDonald, 67 So. 2d 630, 635 (Fla. 1953) ; Miley v. Miley, 402 So. 2d 557, 558 (Fla. 2d DCA 1981). Applying the parol evidence rule to suits for reformation would preclude reformation as a remedy in every case in which the prior agreement was oral, and decisions have held explicitly that the parol evidence rule is inapplicable to suits for reformation. This determination is justified on the basis that equity has the power to afford relief in order to prevent the injustice of binding parties to terms they did not intend and did not agree to, thus overriding the policy underlying the parol evidence rule. Spear v. MacDonald, 67 So. 2d 630, 635 (Fla. 1953).