Johnson, a manufacturer of air conditioning units, made a written contract with Maxwell to sell to Maxwell forty units at a price of $200 each and to deliver them at a certain apartment building owned by Maxwell for installation by Maxwell. Upon the arrival of Johnson's truck for delivery at the apartment building, Maxwell examined the units on the truck, counted only thirty units, and asked the driver if this was the total delivery. The driver replied that it was as far as he knew. Maxwell told the driver that she would not accept delivery of the units. The next day Johnson telephoned Maxwell and inquired why delivery was refused. Maxwell stated that the units on the truck were not what she ordered; that she ordered forty units, that only thirty were tendered, and that she was going to buy air conditioning units elsewhere. In an action by Johnson against Maxwell for breach of contract, Maxwell defends upon the ground that the tender of thirty units was improper, as the contract called for delivery of forty units. Is this a valid defense?