Default_tab 0 not working, when all tab keys are strings!

Hi,

by using the quicktabs as shown below, I realised that the by default set default_tab value 0 is simply not working because there is no tab with key 0.
So, all my tabs where shown. Shouldn't it take the key of the first tab as default_tab when all tabs have keys of the string?!

$tabs = array();
$tabs['overview'] = array('title' => t('Overview'), 'contents' => array('#markup' => $mycontent-x), 'weight' => 0);
$tabs['photos'] = array('title' => t('Photos'), 'contents' => array('#markup' => $mycontent-y), 'weight' => 1);
$tabs['videos'] = array('title' => t('Videos'), 'contents' => array('#markup' => $mycontent-z), 'weight' => 2);
print render(quicktabs_build_quicktabs('qt-test', array(), $tabs));

greetings
ifux