Hi

If you are passing parameter through Action URL, bear in mind that there may be length limitation of URL (http://www.boutell.com/newfaq/misc/urllength.html) and you may pass values that contain lots of blank (when you use multi-select particularly). You can check the URL address to see if it contains a lot of "%20". You may need to trim it so it can go through properly. Try the below method that may help with your problem.

http://www.mssqltips.com/sqlservertip/2844/working-with-multiselect-parameters-for-ssrs-reports/[/url]

Hope this help?

C