Error in using php and 2 terms

I'm using 2 categories and trying to use token to fill automatic nodetitle with the top terms from each category using this code:

  global $form_values;
  return $form_values['taxonomy']['tags'][18]." "$form_values['taxonomy']['tags'][23];
  

It seems to work in creating the concatenated title with the 2 terms, but not without giving me this error.

* warning: Invalid argument supplied for foreach() in /home/mysite/public_html/modules/node/node.module on line 521.
* warning: implode() [function.implode]: Invalid arguments passed in /home/mysite/public_html/modules/node/node.module on line 525.
* user warning: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '' at line 1 query: SELECT n.nid, n.vid, n.type, n.status, n.created, n.changed, n.comment, n.promote, n.sticky, r.timestamp AS revision_timestamp, r.title, r.body, r.teaser, r.log, r.format, u.uid, u.name, u.picture, u.data FROM node n INNER JOIN users u ON u.uid = n.uid INNER JOIN node_revisions r ON r.vid = n.vid WHERE in /home/mysite/public_html/includes/database.mysql.inc on line 172.

I get the error whether I try to us just one term or both concatenated. so it seems more likely due to there being 2 categories. In fact I even got the error without using php but by using a replacment from the token module.

But the title does create ok an dit seems to work fine. So can I just shut off the error message?