Vocabulary Tab desciption is 'null'

None of my tabs have descriptions and all properly get the CSS no description class, but for some reason the "Vocabulary" tab tries to find a description and fails, and outputs the word "null" as a string as the tab description.

For the time being I just commented out the line, but not sure why it's happening.

The line I commented out is in vertical_tabs.js, line 10

        description = '<span class="description">'+ Drupal.verticalTabs[v.callback].apply(this, v.args) +'</span>';

So that's just the output line. Apparently it is passing the test above

      if (v.callback && Drupal.verticalTabs[v.callback]) {

but returning null for the actual apply method.