[Method Error 500] In Cascading Dropdown Update In Ajax Control Toolkit

I am getting [MethodError 500] when I use cascading drop down. below is my code

<tr>
<td >
Select a Hoster:
</td>
<td>
<asp:DropDownList ID="ddlFeaturedHoster" runat="server" ></asp:DropDownList>
</td>
</tr>
<ajaxToolkit:CascadingDropDown ID="cddHoster" runat="server" TargetControlID="ddlFeaturedHoster"
PromptText="Select a Hoster" LoadingText="Loading ..." Category="ActiveHoster"
ServiceMethod="GetDropDownContents" ServicePath="~/Hosting/HostingService.asmx"/>
Service Code:
[WebMethod]
[ScriptMethod]
public CascadingDropDownNameValue[] GetActiveHosters()
{
List<CascadingDropDownNameValue> returnList = new List<CascadingDropDownNameValue>();
HostersManager hosterManager = new HostersManager();
List<Hosters_HostingProviderDetail> hosters = hosterManager.GetAllHosters();
returnList.Add(new CascadingDropDownNameValue("--Please Select One--","0",true));
foreach (Hosters_HostingProviderDetail item in hosters)
{
returnList.Add(new CascadingDropDownNameValue() { name=item.HostingProviderName, value= item.HosterID.ToString()});
}
return returnList.ToArray() ;
}
[WebMethod]
[ScriptMethod]
public CascadingDropDownNameValue[] GetDropDownContents(string knownCategoryValues, string category)
{
knownCategoryValues = FormatCategoryWord(knownCategoryValues);
List<CascadingDropDownNameValue> values = new List<CascadingDropDownNameValue>();
HostersManager hosterManager = new HostersManager();
switch (category)
{
case "ActiveHoster":
values.AddRange(GetActiveHosters());
break;
case "ActiveOffer":
values.AddRange(GetActiveOffers(1));
break;
}
return values.ToArray<CascadingDropDownNameValue>();
}
/// <summary>
/// Formats the category word
/// </summary>
/// <param name="value"></param>
/// <returns></returns>
private string FormatCategoryWord(string value)
{
if (string.IsNullOrEmpty(value)) return value;
if (value.LastIndexOf(":") > 0) value = value.Substring(value.LastIndexOf(":") + 1);
if (value.LastIndexOf(";") > 0) value = value.Substring(0, value.LastIndexOf(";"));
return value;
}
}