CodeIgniter text overlay on transparent png-24 changes background to black

[eluser]wanderer[/eluser]
I am trying to write some text on a transparent PNG-24 image using the following code:

Code:

$config['source_image'] = '/wamp/www/img/test.png';
$config['new_image'] = '/wamp/www/img/test1.png';
$config['wm_text'] = "TEST TEXT";
$config['wm_type'] = 'text';
$config['wm_font_path'] = './system/fonts/texb.ttf';
$config['wm_font_size'] = '11';
$config['wm_font_color'] = '555555';
$config['wm_vrt_alignment'] = 'top';
$config['wm_hor_alignment'] = 'center';
$config['wm_vrt_offset'] = '77';
$config['wm_hor_offset'] = '3';

$this->image_lib->initialize($config);
$this->image_lib->watermark();

but after writing, the transparent PNG background is being changed into black. Any help would be appreciated.