Katy signed a two-year lease containing a clause that expressly prohibited subletting. After six months, Katy asked the landlord for permission to sublet the apartment for one year. The landlord refused. This angered Katy, and she immediately assigned her right under the lease to Jay. Jay was a distinguished gentleman, and Katy knew that everyone would consider him a desirable tenant. Is Katy's assignment of her lease to Jay valid?