Ajaxcontroltoolkit AutocompleteExtender Timeout related

Question

I am trying to use the AjaxControlToolkit Autocompleteextender (for Visual Studio 2008 having ToolkitScriptManager) with values returned from database to be suggested. When I am returning a simple string array it is working fine. But when I am trying to pull data from the database, it is not working. Below is my web service code and the Autocompleteextender Html. When I am running the following without breakpoints no error is thrown but does not work. And when I put breakpoints I am getting javascript error "Microsoft JScript runtime error: Sys.ParameterCountException: Parameter count mismatch."

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
[ScriptService]
public class AutoComplete : System.Web.Services.WebService
{

    [WebMethod]
    [ScriptMethod]
    public string[] GetAgentCompanyAutoCompleteList(string prefixText, int count, string contextKey)
    {
        //return new string[] { "aaaaaaa", "bbbbbb" };

        string sql = string.Format("select distinct AgentCompany from Realtors where FK_CompanyID = {0} Order By AgentCompany", contextKey);
        string[] items = new string[] { "" };
        string cn = "Data Source=localhost;Initial Catalog=ABCD;Integrated Security=True";
        using (SqlDataAdapter da = new SqlDataAdapter(sql, cn))
        {
            DataTable dt = new DataTable();
            da.Fill(dt);
            if (dt != null && dt.Rows.Count > 0)
                for (int i = 0; i < dt.Rows.Count; i++)
                    items[i] = dt.Rows[i]["AgentCompany"].ToString();
        }
        return items;            
    }
}

<asp:TextBox ID="txtCompany" runat="server" MaxLength="256" Width="150px">/asp:TextBox>                                    
<ajax:AutoCompleteExtender 
     ID="AutoCompleteExtender1" 
     runat="server" 
     EnableCaching="true" 
     MinimumPrefixLength="1" 
     TargetControlID="txtCompany" 
     ServicePath="~/AutoComplete.asmx" 
     ServiceMethod="GetAgentCompanyAutoCompleteList" 
     UseContextKey="true">
</ajax:AutoCompleteExtender>