Invalid argument supplied for foreach() in /modules/imagefield/imagefield.module

Hi, when trying to upload a new image using imagefield I get the error above which relates to this part of the code:

/**
 * Validate the widget.
 */
function _imagefield_widget_validate($node, $field, $items) {

  if ($field['required']) {
    // Sum all the items marked for deletion, so we can make sure the end user
    // isn't deleting all of the images.
    $deleted = 0;
    foreach ($items as $item) {
      if ($item['flags']['delete']) {
        ++$deleted;
      }
    }

    if (!count($items)) {
      form_set_error($field['field_name'], t('@field is required. Please upload an image.', array('@field' => $field['widget']['label'])));
    }
    else if (count($items) == $deleted ) {
      form_set_error($field['field_name'],  t('@field is required. Please uncheck at least one delete checkbox or upload another image.', array('@field' => $field['widget']['label'])));
    }
  }
}

And I also figured out that the function _imagefield_widget_validate above was called from the function imagefield_js.

In 90% of the cases I don't get this mistake, but some of the users do get. I tried to replicate it from my home PC and didn't succeed. But I replicated this mistake from my work PC.

Some more details. This mistake does not depend on the browser. I tried with IE6 and FF2 at home - no problem. I tried with the same browsers at work - both fail.

I wonder if this mistake is linked somehow with the VPN/proxy/firewall/other security software installed on my office machine?

I also applied this patch http://drupal.org/node/263637 to add the "minimum resolution" option to the module. Personally, I don't see how it can cause this mistake, but FYI, just in case...

Thank you for your support!