geoff5,

This report started it's like as a SSRS 2005 report. I believe I started it with VS 2008 or earlier (can't remember now). I've migrated it to VS 2010, using .NET 4.0, so I'm thinking it might be a 2008 R2 report now. Also, although I didn't say it before, this report exists as a .RDLC report within the application. Our database is currently a SQL Server 2005 database (we're working on upgrading it, but that's going to take some time). My point is that this report runs within the context of a .NET application, not within SSRS.

However, your whole point of the Lookup() function not being introduced until SSRS 2008 R2 makes me wonder. Within the designer, when I look at the expression, there's a red wavy line under Lookup. Could that be because, even though the .RDLC is within a .NET 4.0 app, it is still hitting against a SQL 2005 database?