Sort input cells on dhtmlxGrid v.1.5


I’m working on a project that uses dhtmlxGrid v.1.5 build 71114.

What I’m trying to do is to sort a column that contains input fields in cells.

<table id="norprice">
<td width="100%">Normal Price</td>
<tr class="light-color">
<td align="left"> <input name="nprice2" type="text" value='500' size="10"  /> </td>
<td align="left"> <input name="nprice5" type="text" value=450' size="10"  /> </td>
<td align="left"> <input name="nprice6" type="text" value=' ' size="10"  /> </td>
<td align="left"> <input name="nprice4" type="text" value='550' size="10"  /> </td>
<td align="left"> <input name="nprice8" type="text" value='370' size="10"  /> </td>
<td align="left"> <input name="nprice3" type="text" value=' ' size="10"  /> </td>

The issue is that the sorting is working only for the first two cells.
If I remove the and use something like

500 the sort is working fine for all the cells.

What can I do to fix this, even if this is a older version of dhtmlxGrid?