[wpmem_txt] prints itself when I echo login shortcode

Hi
First of all I am happy to use this plugin.It works well but when I echo the log in form short code in a page, that time “[wpmem_txt]” is print automatically before and after the form.

<?php echo do_shortcode(‘[wpmem_form login]’)?>

How can I remove that,please let me know.

Thanks in advance.