OCUM Reporting - Computed Column trouble - left([Volume],3)


I'm teaching the OCUM Administration course right now and one of my students wanted to group a volume report according to customers. The customer nick is always the first three letters of the volume name. I suggested to create a computed column with the customer nick (and then group according to this column):


Validation always fails with 'no column with that name', when it's clearly visible and looks like it's exactly this name ("Volume").

We're on 

Edition: Standard edition of DataFabric Manager server
Version: (5.0.2)

Any ideas why it doesn't work?