Andy has not granted a valid sublease and the transaction operates as an assignment of his lease. For Andy to grant a lease, he must retain a "reversionary" interest in 25 The Street, meaning that any sublease he grants should be shorter than his own lease term. Since Andy's solicitor erroneously drafted the sublease term to be equal to the remaining term of Andy's lease, no reversionary interest is retained. Consequently, the transaction operates as an assignment of Andy's lease by operation of law (Beardman v Wilson (1868) LR 4 CP 57). 

Key Point: When granting a sublease, the original tenant must ensure the sublease is for a term less than their remaining lease term to retain a reversionary interest. Failure to do so results in the transaction being treated as an assignment rather than a sublease.