If empty token is used php breaks

Hi, I use [node:source:nid] tokens to generate automatic node titles in a multilanguage-web-site.

As long a node has already a translation, everything works fine.
But, if there is no translation, the node-title is empty.
Interesting part: Even if the "empty" token isn't part of the returned value, the node title is empty.

$var_pro = '';
$var_sourcenid = [node:source:nid]; 'breaks the node title when there is no translation
$var_lang = [node:language];
if ($var_lang == 'fr') {
    $var_pro = 'Projet';
} elseif ($var_lang == 'en') {
    $var_pro = 'Project';
} else {
    $var_pro = 'Projekt';
}
$title = $var_pro . '#10';
return $title;

I tried to proof if the token exists (!empty ....) - but always the node title is empty. Can anyone help?