Internal row index in DataTables rowCallback

Question

The doc of DataTables rowCallback claims that the fifth argument of the function is the internal row index. However when I use this row callback below, I get undefined for dataIndex. How to use the internal row index in rowCallback ?

  function(row, data, displayNum, displayIndex, dataIndex){
    console.log(displayNum); console.log(displayIndex); console.log(dataIndex);
  }