problem with search query string

[nomu26]

I have created a form to perform a search with dropdowns and textboxes, I want the user to select any search criteria, if all is selected it works fine but if you select some of the fields it return an error :Microsoft JScript runtime error: object doesn't support this property or method, here's the code:

<script language="javascript" type="text/javascript">
var wMyWindow;
function MoreInfo(sValue)
{
sValue = sValue +'?qstrEntityCode=' + document.frmSalesReport.cboEntity.options[document.frmSalesReport.cboEntity.selectedIndex].value + '&qstrEntity=' + document.frmSalesReport.cboEntity.options[document.frmSalesReport.cboEntity.selectedIndex].text + '&qstrVolumeNo=' + document.frmSalesReport.cboVolumeNo.options[document.frmSalesReport.cboVolumeNo.selectedIndex].value + '&qstrUseCode=' + document.frmSalesReport.cboUseCode.options[document.frmSalesReport.cboUseCode.selectedIndex].value + '&qstrRateCode=' + document.frmSalesReport.cboRateCode.options[document.frmSalesReport.cboRateCode.selectedIndex].value + '&qstrHomogeneousCode=' + document.frmSalesReport.txtHomogeneousCode.value + '&qstrBlockPlan=' + document.frmSalesReport.txtBlockPlan.value;
var bName = navigator.appName;
var bVer = parseInt(navigator.appVersion);
if (sValue.length > 1)
{
if (wMyWindow && (((bName.indexOf("Netscape") > -1) && (bVer > 2)) || ((bName.indexOf("Explorer") > 0) && (bVer > 2))))
{
if (!wMyWindow.closed)
{
wMyWindow.close();
}
}
wMyWindow = window.open(sValue ,'_blank');
}
}
</script>
<form>
<TR>
<TD align="center" bgColor="#9cb0be" colSpan="2"><A href="javascript:MoreInfo('SalesAnalysisReportView.aspx');"><IMG id="IMG1" alt="Submit" src="../images/submit.gif" align="absMiddle" border="0" runat="server"></A>
</TD>
</TR>
</form>

if someone can tell me what's wrong with my query string cause i've even tried testing one by one it still gives me the error but if I select everything it returns no errors.

thanks
cheers