Dashboard logo edit

Hello,
i try to edit WP dashboard logo (upper left corner) but it don’t work. What should i fix?

// Dashboard Logo
add_action('admin_head', 'custom_admin_logo');
function custom_admin_logo() {
    echo '
        <style type="text/css">
            #header-logo { background-image: url('.get_bloginfo('stylesheet_directory').'/img/dashboard-logo.png) !important; }
        </style>
    ';
}