jQuery Grid Plugin - jqGrid

Hi,

I using the latest JQGrid (3.5.3) and I need to disable a select list (dropdown) in my edit form during edit operations. Reason being is that the field to be disabled is part of the primary key and should not be changed during edit. The same field should be 'selectable' during add operations. The code below make my select list disabled however the form will not display on the first click of the edit icon on the nav bar. Strangely - it does come up on the next click of the edit icon and the dropdown is disabled as it should be. Is this a bug? Or is there a better way to achieve what I want. Thanks.

beforeShowForm: function(eparams) {

             document.getElementById('equip_id').disabled = true;
             document.getElementById('service_dt').disabled = true;
             document.getElementById('serv_descr').disabled = true;
             document.getElementById('calc_next_svc').checked = 'true';
         }

This is in my edit options array of the navGrid method. The dropdown is 'serv_descr'. The other 2 are text fields and do not cause this issue. Only the select list disabling causes the problem.