Basic Mass Update deluge schedule not working

Basic Mass Update deluge schedule not working

I'm trying to create a schedule that will 'reset' a single field to 0 every morning - so that another schedule can repopulate with the day's calculation. I thought this would be fairly simple but I can't work out why this is failing :

1) I'm based in UK so am using the Eu endpoint
2) The field is a standard currency with no formula or dependencies
3) The CVID is for my main All Account view
4) My connection has all the scopes for MassUpdate selected 

  1. void schedule.Sales_Rollup_Reset()
  2. {
  3.     dataMap = Map();
  4.     dataMap.put("Sales_Invoice_Value_12", 0);

  6.     dataList = List();
  7.     dataList.add(dataMap);

  9.     requestMap = Map();
  10.     requestMap.put("data", dataList);
  11.     requestMap.put("cvid", "2726580000000XXXXX");

  13.     response = invokeurl
  14.     [
  15.         url :"https://www.zohoapis.eu/crm/v8/Accounts/actions/mass_update"
  16.         type :POST
  17.         body : requestMap.toString()
  18.         headers : {"content-type":"application/json"}
  19.         connection : "MYCONNECTION"
  20.     ];
  21.     info response;
  22. }
info response gives me :

{"data":[{"code":"INVALID_DATA","details":{"api_name":"Sales_Invoice_Value_12","json_path":"$.data[0].Sales_Invoice_Value_12"},"message":"This field cannot be updated in the Mass Update","status":"error"}]}

I welcome any insight !