How to use a RTP in substring?

Summary:

I'm trying to reference a calc manager RTP as an argument for a substring but getting a data type error.

First as an example, if I use a hardcoded value like so it works:

FIX(@MEMBER(@SUBSTRING(@NAME("P13-FY22"),4)))

But if I create an RTP of data type "string" and use it like this I get an error:

FIX(@MEMBER(@SUBSTRING(@NAME({CDM_Year}),4)))

Error:

Error:Error parsing formula for [FIX STATEMENT, at or after line: ["FIX(@MEMBER(@SUBSTRING(@NAME(P13-FY22),4))"]] (line 5): expected type [MEMBER | STRING | LIST] found [NUMBER] ([@SUB]) in function [@NAME]

The RTP is a string by definition so what am I missing?