(RS&FM) Issue with Calculated text field

[Arsenal]

Hi all,

In FM, I'm adding a calculated query item which takes codes and converts it to text

So, it's basically:

case

when code = '00001' then ('A')
When Code = '00002' then ('B')

and so on

Problem is, when a filter is set as an optional prompt for this query item in RS, I keep getting a sql error...but if I just add this to the report body without the filter, it works fine.

Any ideas? Thanks