Reporting Service Report fields cannot be used in page headers of footers

http://blogs.technet.com/b/mat_stephen/archive/2005/04/27/404209.aspx

Reporting Services doesn't like you using data fields in its headers and footers - here are two ways to work around the problem

Add a parameter to the report without a prompt (so the user doesn’t see it) and with a default value based on the field. Use this parameter instead of the field in the page header.

Put a textbox in the body of the report that references the field. Reference the value of the textbox in the page header. Optionally hide the textbox. (Note: You’ll need to make sure the textbox appears on every page. A couple of ways to do this is
either RepeatOnNewPage in a table header or RepeatWith)