Arbitrary groupings of rows

I have Community Codes that are AlphaNumeric that I wish to group into Regions (e.g. 100, 101, 102, 106 in region "SS", 105, 107, 110 in region "NS", and others.) I tried using Row Group | Group Properties, Group On Expression 

  =iif(Fields!Location.Value in ("100","101","102","106"),"SS","OTHER")

I received the error message, in part,

  Argument not specified for parameter 'TruePart' of 'Public Function IIF(Expression As Boolean, TruePart As Object, FalsePart As Object'.

I don't see how "SS" is not a valid option for TruePart of the IIF expression. Can someone help me out?

Thanks, j