Doubts in .length function

In my case I'm picking up how many records are inside the variable. But the .length function determines that when there is only one record it is undefined , and it should show that there is a record in it. Is this some kind of function error, or is there a way to fix it? Or is there some other method I can use?

I'm storing this way:

function insert_info(){
    w_qtde_info = document.forms['my_form'].blc_info.length;

}

    <form name="my_form"  method="post">
    <table width="100%">
    <tr>
    <td>
<?php
        while($w_cont <= $w_opc)
        { 
        print('<input type="radio" name="blc_info" value="">');
         } 
        print('<input type="button" onClick="insert_info();" value=">>">');
?>
    </td>
    </tr>
    </table>
    </form>

w_opc is determined by the user. That is, he will add the radios (according to the example) according to the value that the user has determined!

Obs: I took the part of php as a reference for: Text radio