ajax error {"Message":"There was an error processing the request.","StackTrace":"","ExceptionType":""}

I calling a local asp.net web service via an jquery ajax call.  I'm getting an error, which in Fiddler comes back as {"Message":"There was an error processing the request.","StackTrace":"","ExceptionType":""}.   I have several other web services similar to the one giving an error that work fine, the only difference between them is the one in question returns a string value.  I can run the web service from visual studio and it works fine.  I can run the stored procedure that's called in the web service and it works fine.  I thought it might have been a problem with the data syntax in the ajax call so I hard coded the value in web service.  I've changed the entity framework function import call to return both "none" and "string" without change. I did remember to uncomment the json call in the web service. At this point I'm at a loss.

I would appreciate it if someone could look at the code and see if there are any glaring mistakes or offer some advise of what else to check.

Thanks,
Tom

[System.Web.Script.Services.ScriptService]
public class wsDeptServices : System.Web.Services.WebService {
    sfhsEntities ctx = new sfhsEntities();

    public wsDeptServices () {

        //Uncomment the following line if using designed components
        //InitializeComponent();
    }

    [ScriptMethod(ResponseFormat = ResponseFormat.Json)]

    [WebMethod]
    public String GetApparatusStationLocaion(int ID)
    {
        using (sfhsEntities context = new sfhsEntities())
        {
            System.Data.Objects.ObjectParameter AoutputParameter = new System.Data.Objects.ObjectParameter("StationDesc", typeof(String));
            ctx.GetApparatusStationLocation(ID, AoutputParameter);
            return (String)AoutputParameter.Value;
        }
    }
    
}

 var pageUrl = '<%=ResolveUrl("~/wsGetDefCount.asmx")%>'
                     $.ajax({
                         type: "POST",
                         url: pageUrl + "/GetApparatusTroubleTicketCountByID",
                         data: "{ 'ID': '" + $("#txtCurID").val() + "' }",
                         contentType: "application/json; charset=utf-8",
                         dataType: "json",
                         async: false,
                         success: function (msg) {
                             //alert('In ' + msg.d);
                             $("#txtTTID").val(msg.d);
                         },
                         error: OnError
                     });