What's the exception message?
When I inserted that expression into one of my reports (substituting my report's fields of course), it ran without incident.
Leads me to believe it's a problem related to your fields somehow. Check your spelling, case, punctuation, etc. SSRS is picky and unforgiving.
[font="Comic Sans MS"]toolman[/font]
[font="Arial Narrow"]Numbers 6:24-26[/font]