Null column serverside search error

Restful Web Services

I am receiving the following error message when I turn on Serverside processing, Unknown Field : NULL (index0)

It is because my first column is a selectable checkbox which isn't a real column in my database. I have used php "data": "NULL", defaultContent: '', orderable: false to signify the column is empty and is supposed to be but it is not working as I would expect when I attempt to use the search function. Can someone give me any indication how to correct this?

        "columns": [
            { 
                "data": "NULL", defaultContent: '', orderable: false  
            },
            {
                "data": "listphone"
            },
            {
                "data": "create_date"
            },
            {
                "data": "checkcode"
            },
            {
                "data": "registered_with_tps"
            }
        ],

<table cellpadding="0" cellspacing="0" border="0" class="display" id="cms_module_tps_userlisting" width="100%">
        <thead>
            <tr>
                <th> </th>
                <th>Number</th>
                <th>Added</th>
                <th>Test Result</th>
                <th>Found on TPS</th>
            </tr>
        </thead>
 
        <tfoot>
            <tr>
                <th> </th>
                <th>Number</th>
                <th>Added</th>
                <th>Test Result</th>
                <th>Found on TPS</th>
            </tr>
        </tfoot>
</table>

Many Thanks

Chris

Restful Web Services

Found the answer,

        "columns": [
            { 
                "data": "null", defaultContent: '', orderable: false, "bSearchable": false
            }
         ]