I am trying to utilize this $find syntax in my Javascript to access the selected index in my WebDropDown, but I keep getting this error, "Microsoft JScript runtime error: Sys.ArgumentTypeException: Object of type 'Object' cannot be converted to type 'String'.  Parameter name: id".  This is what the markup for the WebDropDown looks like on my page:

<ig:WebDropDown ID="lstPractice" runat="server"  EnablePaging="false" PageSize="10"
EnableRenderingAnchors="false" ForeColor="Black" DataSourceID="lstPracticeSqlDataSource"
AutoPostBack="true"  Visible="true" Width="375"  selectedValue='<%# Bind("dept_cd") %>'
OnSelectedIndexChanged="lstPractice_OnSelectedIndexChanged" >
  <DropDownItemBinding TextField="dept_desc" ValueField="dept_code" />
</ig:WebDropDown>

This is what my Javascript code looks like that is attempting to get the WebDropDown's selected item index:

function OnPageLoadProcess2ndaryPractDDL() {
 var CallingDDL_ID;
 var selText;
 var selValue;
 var selIndex;

 CallingDDL_ID = FormView1 + '_lstPractice';
 var CallingDDL = document.getElementById(CallingDDL_ID);

--The following line of code is what generates the error!
 selIndex = $find(CallingDDL).get_selectedItemIndex(); 

 if (selIndex != -1) {
   selValue = CallingDDL.options[selIndex].value;
   selText = CallingDDL.options[selIndex].text;
 }
else
{
   selValue = -1;
   selText = -1;
 }
}

What is the proper syntax to get the WebDropDown's selected index?  What is generating the error?

Thank you,
Chris