Method error 500 ......... plz help

Can any tell why is it happening
 
thanks
 
[WebMethod]
public CascadingDropDownNameValue[] GetData_fields(string knownCategoryValues, string category)
    {

        // string[] _categoryValues = knownCategoryValues.Split(':', ';');
         //string _fieldID = Convert.ToString(_categoryValues[1]);
        fields_DataSet1TableAdapters.FieldsTableAdapter fieldsadaptor = new fields_DataSet1TableAdapters.FieldsTableAdapter();
        List<CascadingDropDownNameValue> fieldsvalues = new List<CascadingDropDownNameValue>();

        foreach (DataRow rows1 in fieldsadaptor.GetData_fields())
        {
         ...