A nonsense mutation is the one which stops polypeptide synthesis due to the formation of termination or non-sense codon. In lac operon, sequence of structural genes is Z, Y and A, which respectively code for $\beta$-galactosidase, lactose permease and transacetylase. If the gene Y has nonsense mutation, gene expression will stop at it, resulting in non-expression of both gene Y and successive gene A. Thus, only $\beta$-galactosidase enzyme will be produced.