Custom Dashboard RSS Feed Strict Error

I’ve recently added a custom RSS feed dashboard widget to one of my plugins and I see the following Strict message:

call_user_func_array() expects parameter 1 to be a valid callback, non-static method WP_Feed_Cache::create() should not be called statically on line 215 in file /path/to/wp-includes/SimplePie/Registry.php

The offending call is:

wp_add_dashboard_widget( $widget_id, // widget slug
     $this->widget_options[$widget_id]['title'], // title
     array(&$this, 'whats_new_rss'), // display function
     array(&$this, 'whats_new_rss_control') // control function
 );

public function whats_new_rss()
{
    $widgets = $this->get_widget_options();
    $value = @$widgets[self::WHATS_NEW_RSS_ID];

    echo '<div class="rss-widget">';
    wp_widget_rss_output( $widgets[self::WHATS_NEW_RSS_ID] );
    echo "</div>";
}

If I comment out wp_widget_rss_output() the message stops showing (and of course I don’t get my RSS output).

Why does this happen with my call but not with the core dashboard RSS widgets?

Cheers,
Vinny