There is indeed a bug regarding that case on the current release.
We were able to reproduce it on our end and fixed. The fix will be included in next release.
In the mean time you can do this:
In the file components/com_hikashop/controllers/checkout.php, after the line:
$done = $cartClass->loadFullCart(true);
add the code:
if(empty($done->products)){
$app =& JFactory::getApplication();
$app->redirect( $this->redirect_url, JText::_('CART_EMPTY'));
}
Then, in the file administrator/components/com_hikashop/classes/cart.php, you need to change the lines:
$notUsable[$k]=0;
by:
$notUsable[$product->product_id]=0;
You should have it 4 times.
That solves the problem on our end.