two rows of th save to excel

My table's thead has two rows of th:
<thead>
<tr><th rowspan="2">X</th><th colspan="2">Y</th></tr>
<tr><th>Y1</th><thY2</th></tr>
</thead>
I use tabletools to save to excel.But in the excel only has one row of th:X Y1 Y2.Y is missing.It can't save the all two rows of th.Someone who can help me.Thank you.