Invalid colspan issue

The following does not validate.

         <table>
             <tr>
                 <td colspan="6">1</td>
             </tr>
             <tr>
                 <td colspan="3">2</td>
                 <td colspan="3">3</td>
             </tr>
             <tr>
                 <td colspan="2">4</td>
                 <td colspan="2">5</td>
                 <td colspan="2">6</td>
             </tr>
         </table>

I am trying to create a table like this.

I am getting the following message on the colspan="6" line.

/*Table column 2 established by element *//*|td|*//*has no cells 
beginning in it*/

I am assuming it is saying that there are no tds beginning on column 2, 
and there aren't, but this still should validate in my opinion.

It used to validate under XHTML.