Error -emogrifier.php on line 100

This error was in my Drupal Log. I do not understand the message, so I do not know what to check or change in my settings. It had not appeared until recently. Would this be an error within the newsletter message content or the heading/footer content?

• Type: php
• Location: http://ourdomain.org/cron.php
• Message:
  DOMXPath::query() [domxpath.query]: Invalid expression in /path/to/sites/all/modules/simplenews_template/emogrifier.php on line 100.
• Severity: error