Hiding file upload causes function impode error

At first I thought this was an ubercart issue, but I've tested it on my site for standard CCK image and file nodes and I get the same error. This seems like such a simple error and other people don't seem to be having it, so I'm uncertain what's going on here.

If I attempt to hide a file upload field by clicking "Hide upload", when I go to preview the form the upload is not hidden and I get the following error:

warning: implode() [function.implode]: Invalid arguments passed in (my site directory)/drupal/includes/form.inc on line 837.

form.inc line 837 is the implode line of:

/**
 * Return the error message filed against the form with the specified name.
 */
function form_get_error($element) {
  $form = form_set_error();
  $key = $element['#parents'][0];
  if (isset($form[$key])) {
    return $form[$key];
  }
  $key = implode('][', $element['#parents']);
  if (isset($form[$key])) {
    return $form[$key];
  }
}

I tried throwing some drupal devel dpm in there to investigate the $element map, and $element['parents'] appears to contain three elements: field_my_file (the field name), 0, upload

My experience tells me this is something I've messed up (the server is heavily loaded with modules, perhaps there's a conflict?), but I thought I'd check here to see if anyone had any ideas.

Thanks in advance!
Trevor Bradley