OBIEE Web Servcies - ReportParams

 I have successfully integrated with OBIEE's Web Services to pull back HTML for specific reports. I am now moving on to filtering the reports using the ReportParams structure. I have found virtually no documentation on how to do this. I would essentially like to pass through a parameter inside of the htmlViewService object in order to filter a report. I have something like this:

ReportParams reportParams = new ReportParams();

reportParams.filterExpressions[2] = "3050";

obiconn.m_HTMLservice.addReportToPage(pageID, reportName, reportRef, null, reportParams, null, sessionID);

I am getting an error: Object reference not set to an instance of an object

I'm assuming that I am not using the filter expression array properly. Can someone please help?