Compile an Iif statement with following conditions

[rlsublime]

I am trying to compile an IIF statement with the following conditions but I cannot get the query to run for some reason. any idea what i have done wrong here?



LnPreApprConvDt:iif (LnPreApprFg is null,LnPreApprConvD,is null)

Thanks

[help_me_with_access]

LnPreApprConvDt:iif ([LnPreApprFg] is null,[LnPreApprConvD],NULL)

LnPreApprConvDt:iif ([LnPreApprFg] = NULL,[LnPreApprConvD],NULL)