Can't join on calculated field in Domain Designer ?

I use JasperReports Server Pro 5.0.1

I want to join 2 tables, of which one field is a calculated field (made in domain designer)
Apparently, this cannot be done, as the join-icons grey out when I select my calculated field.

Why is this ?
I tried to work around it, and make a derived table from my entire table, plus the calculated field, but that completely slows everything down.
I'm under the impression derived tables don't use indexes.

Another workaround would be to physically include the calculated field in my table, but I'm no fan of that.

Any suggestions, or is this just not possible (and why ??).

Thanks on beforehand.