Rest API 1.1 - Authorization fails if parameter added to query string


I have a application running successfully with User Timeline and certain parameters.

If I add since_id to the list of parameters in the query, the foolowing error is returned “The remote server returned an error: (400) Bad Request.”

If i remove the since_id from the parameter list, it works.

What do I need to do to add this parameter to my query?

Thanks in advance