Can not read property .set of undefined ...

I keep getting the error "Cannot read property .set of undefined" when I try to update a field from a button. Code is as follows ...

      {
        extend: 'selectedSingle',
        text: 'Change Last Name',
        action: function(e, dt, node, config) {
          editorMain
            .edit( table.row( { selected: true } ).index(), false )
            .set( 'LastName', 'Some Last Name' )
            .submit();
//           editorMain
//             .remove(table.row( { selected: true } ).index(), false)
//             .submit();
        }

If I toggle the comments on the ".remove" and ".set" code, the record is faithfully deleted but I can't seem to make the ".set" code work.

What am I missing?

Dave