"A mutual mistake is a mistake common to both parties." Mitchell v. First Nat'l Bank in Stuttgart, 293 Ark. 558, 560, 739 S.W.2d 682, 683 (1987). "Before a mutual mistake will affect the binding force of a contract, the mistake must be of an existing or past material fact which is the basis of the contract." Id. Further, "although parol evidence is generally not admissible to vary the terms of a written instrument, it is admissible to show mutual mistake." Garot v. Hopkins, 266 Ark. 243, 244, 583 S.W.2d 54, 55 (1979). Because of the mutual mistake of material fact, the contract was unenforceable and the granting of summary judgment was proper. See Carter v. Matthews, 288 Ark. 37, 701 S.W.2d 374 (1986)(mutual mistake where both parties thought that the low, flat portion of land was suitable for building permanent structures when the area was subject to severe and frequent flooding); Blythe v. Coney, 228 Ark. 824, 310 S.W.2d 485(1958)(mutual mistake where parties assumed there was a sufficient water supply to the house); First Nat'l Bank of Wynne v. Coffin, 184 Ark. 396, 42 S.W.2d 402 (1931) (mutual mistake where neither the buyer nor seller knew the exact acreage of farm property or that it had been platted and subdivided with streets dedicated to the city).