AJAX :: AutoCOmpleteExtender Stopped Working When Added Parameters

I have added autocompleteextender in page . It was working without parameters , when i have added parameters "pretext","count" it stops working. below are my code :

<div>
<asp:ToolkitScriptManager
ID="ToolkitScriptManager1"
runat="server">
<Services>
<asp:ServiceReference
Path="~/Admin/WebService1.asmx"
/>
</Services>
</asp:ToolkitScriptManager>
<asp:TextBox
ID="TextboxCompletelist"
runat="server"></asp:TextBox>
<asp:AutoCompleteExtender
ID="TextboxCompletelist_AutoCompleteExtender"
runat="server"
ServiceMethod="GetCompletionList"
Enabled="True"
ServicePath="~/Admin/WebService1.asmx"
TargetControlID="TextboxCompletelist"
CompletionSetCount="10">
</asp:AutoCompleteExtender>
</div>
namespace AgentService
[WebService(Namespace = [URL]
[WebServiceBinding(ConformsTo =
WsiProfiles.BasicProfile1_1)]
[ToolboxItem(false)]
[System.Web.Script.Services.ScriptService]
public
class
WebService1 : System.Web.Services.WebService
{
[WebMethod]
public
static
string[] GetCompletionList(string PrefixText,
int count)
{
string countryname;
SqlParameter[] parm =
new
SqlParameter[1];
parm[0] = new SqlParameter("@PrefixText",
PrefixText);
DataTable dt =
SqlHelper.ExecuteDataset(AppSetting.ActivateConnection,
CommandType.StoredProcedure,
"FetchAutoCompleteStates",parm).Tables[0];
List<string> items =
new
List<string>();
for (int i = 0; i <= dt.Rows.Count-1; i++)
{
countryname = dt.Rows[i]["statename"].ToString();
items.Add(countryname);
}
return items.ToArray();
}