Common table expression defined but not used.

Question

Getting following error

 

Msg 422, Level 16, State 4, Line 9

Common table expression defined but not used.

 

with vals
as
(
select I_QTY as Iss  From tbl_Issuance where I_Type='Issue'
union all
select I_QTY as Ret1  From tbl_Issuance where I_Type='Return'
)

select Iss - Ret1  as difference