Conditional formatting in SSRS based on presence in list

I'm trying to set a background color for an SSRS field based on the presence of the field's current value in a list containing ~40 items. I'd like to use the syntax =IIF(Fields!Pt_type.Value IN ('Inpatient','Outpatient','ER','OB'), "LightGrey","Transparent"), but that throws an error. The list is much longer, so I'd like to find a clean way to accomplish this.