foreach() problem in jsmenu

"Invalid argument supplied for foreach() in .../drupal/modules/taxonomy_jsmenu.module on line 51"

Can anybody tell me what this means?
I have some taxonomies set up and when I enable the taxonomy_jsmenu module I get this message. at the top of the screen, and no menu blocks are created. Does anybody know what I'm not doing? This module seems to have worked for everybody else!

line 51 reads foreach ($tree as $term) {