warning: implode() for theme_textarea

I'm getting the following warning when I view a page with a textarea form item:

warning: implode() [function.implode]: Bad arguments. in /var/www/html/sloan51/test/includes/form.inc on line 618.

In my module I have a method that spits out html.

  $properties = array(
    '#title' => t('Email body'),
    '#description' => t('Enter something.'),
    '#rows' => 7,
    '#cols' => 55,
  );
  $output = theme_textarea($properties);
  
  return $output;

I've tried removing all properties and setting all properties and I still get the same warning message. I've tried other theme_xxx method(s) and they work without throwing any warnings, such as:

 $output = theme_item(array('#title'=> 'hey', '#value' => ''));

Anyone had the same problem before?