While on a fishing trip, Tom Snyder met an elderly couple living in near-destitute conditions in a rural area of Texas. He returned to the area often, and he regularly purchased groceries for the couple and paid for their medical needs. Some two years later, the couple's son, David, discovered what Tom had been doing and promised to reimburse Snyder for what he had furnished his parents. He failed to do so and Tom sued David for breach of his promise to reimburse Snyder. Tom has receipts for most of the purchases. What defense, if any, does David have? Decide.