Error when exporting to excel only

I get an error when I export my report to excel. But I do not get this error if I export the report to any other format. The error is:

An error occurred during local report processing.
An error occurred during rendering of the report.
An error occurred during rendering of the report.
Object reference not set to an instance of an object.

Any ideas what could be causing this error?

This report does contain a couple of sub-reports.

This may help you:

http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=790946&SiteID=1