What's the value for sticket and eticket when you get an error?

>hi all,
>
>i try this code under ,
>sometime work, somtime not

>lnSticket = Val(SubStr(csrMyCursor.sticket, 4))
>lnEticket = Val(SubStr(csrMyCursor.eticket, 4))
>replace noofticket with lnSticket-lnEticket
>