Changing row order in a table - prevent changing the header

Question

the code below allows reordering of table rows (moving a row up or down)

for some reason, the following code in IE11 allows changing the header row as well , although i specified "tbody tr:first" as my filter what am I doing wrong?

  function upAction(){
   var row = $("input[name='select_radio']:checked").parents("tbody tr:first");    
   row.insertBefore(row.prev());
  }
  
  function downAction(){
   var row = $("input[name='select_radio']:checked").parents("tbody tr:first");
   row.insertAfter(row.next());
  }

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>

 <button onclick="upAction()">Up</button>
 <button onclick="downAction()">down</button>
<table  >
    <tr>
       <th>Printer</th>
       <th>Printer ID</th>
       <th>Printer Description</th>
    </tr>
    <tbody>
              <tr>
                <td><input type="radio" name="select_radio"/></td>
                <td>x300</td>
                <td>3</td>
                <td>new printer installed in 3th floor</td>
              </tr>
              <tr>
                <td><input type="radio" name="select_radio"/></td>
                <td>x400</td>
                <td>5</td>
                <td>laser printer</td>
              </tr>
              <tr>
                <td><input type="radio" name="select_radio"/></td>
                <td>Office jet 3</td>
                <td>6</td>
                <td>old student printer</td>
              </tr>
  
              </tbody>
   </table>