[imp] getMIMEId() problem?

[li at elaboratio.com]

When I click on a message that contains a receipt, I get the error message

"Fatal error: Call to a member function on a non-object in
/usr/local/www/horde/lib/Horde/MIME/Contents.php on line 911"


This is: 

function urlView(&$mime_part, $actionID, $params = array(), $dload = false)
    {
        /* Get the MIME ID for this part. */
        $id = (isset($params['id'])) ? $params['id'] :
$mime_part->getMIMEId();

        /* Add the necessary local parameters. */
        $params['actionID'] = $actionID;
        $params['id'] = $id;
        $params = array_merge($params, $this->cacheIDURLParam());

        if ($dload) {
            $url = Horde::downloadUrl($mime_part->getName(true, true),
$params);
        } else {
            $url = Util::addParameter(Horde::applicationUrl('view.php'),
$params);
        }

        return $url;
    }


The attachment in .txt contains the following:

Final-Recipient: rfc822;myemail-adress
Original-Message-ID: <20050228042830.y41kp4ltes008880mydomain>
Disposition: manual-action/MDN-sent-manually; displayed



some system data:
FreeBSD 4.10
PHP 4.3.10
Horde 3.0.2 
Imp 4.0.1


Could there be something wrong with my mime configurations?