Filtering Out Nulls in SSRS

Below is my expression, which would work perfectly if the "Is Not Nothing" was legit syntax... I can't figure out how to not include Nulls, is there a way to do this?

=CountDistinct(IIF(
    Fields!StateCode.Value<>"NC"
    AND Fields!StateCode.Value<>"SC"
    AND Fields!StateCode.Value<>"NY"
    AND Fields!StateCode.Value<>"VA"
    AND Fields!StateCode.Value Is Not Nothing
    , Fields!ClientID.Value
    , Nothing))