Error: Undefined variable

I was using devel Backtrace to to some debugging on a site and noticed this error come up:

notice: Undefined variable: output in taxonomy_intro_help() (line 80 of /home/hrstage/public_html/sites/default/modules/taxonomy_intro/taxonomy_intro.module). =>

    ... (Array, 10 elements)
    Krumo version 0.2a
    | http://krumo.sourceforge.net
    Called from /home/public_html/sites/default/modules/taxonomy_intro/taxonomy_intro.module, line 80  

The code around line 80 is:

switch ($section) {
    case 'admin/help#taxonomy_intro':
      $output .= t('<p>This module can be used to output an intro of the current taxonomy term at term pages.</p>');
      break;
  }

  return $output;
}

... with line 80 being the last line I have above.

I am wondering if there is any indication why that is happening. Thanks.