tyring to update a custom field attached to a contact (C#)

I've created a number of custom fields that are attached to Contacts. I'm tyring to figure out how to update one of those custom fields via web services, but I'm getting "An unexpected error has occured" message (which naturally isn't very helpful <g>)

Here's my code (C#, not php - but php is colorful):

public static bool updateContactField(string fieldName, string fieldValue, string contactID, out string statusDetail)  {              StringBuilder sb = new StringBuilder();                DateCustomFieldRef dateCF = new DateCustomFieldRef();              dateCF.internalId = fieldName;              dateCF.value = DateTime.Parse(fieldValue);                CustomFieldRef[] custFieldRefs = new CustomFieldRef[1];              custFieldRefs[0] = dateCF;                Contact contact = new Contact();              contact.customFieldList = custFieldRefs;              contact.internalId = contactID;                WriteResponse wr = misc._service.update(contact);                if (!wr.status.isSuccess)              {                  foreach (StatusDetail sd in wr.status.statusDetail)                  {                      sb.Append(sd.code + "#" + sd.message + "#" + sd.type);                  }              }              statusDetail = sb.ToString();                return wr.status.isSuccess;  }