Fatal error on upload

When users try to upload files using Filefield, they get the following message.

An error occurred:

<br> <b>Fatal error</b>: Cannot pass parameter 1 by reference in
>b>/var/www/html/drupal/sites/all/modules/filefield/filefield.module</b> on line <b>852</b> <br>

Here's the function that contains line 852:

function filefield_js() {
  // Parse fieldname from submit button.
  $matches = array();
  foreach(array_keys($_POST) as $key) {
    if (preg_match('/cck_filefield_(.*)_op/', $key, $matches)) {
      $fieldname = $matches[1];
      break;
    }
  }

  $node = (object)$_POST;
  $field = content_fields($fieldname, $node->type); // load field data

  // Load fids stored by content.module.
  $items = array();
  $values = content_field('load', $node, $field, $items, FALSE, FALSE);
  $items = $values[$fieldname];

  // Load additional field data.
  filefield_field('load', $node, $field, $items, FALSE, FALSE);

  // Handle uploads and validation.
  _filefield_widget_prepare_form_values($node, $field, $items);
  _filefield_widget_validate($node, $field, $items);

  // Get our new form baby, yeah tiger, get em!
  $form = _filefield_widget_form($node, $field, $items);

  foreach (module_implements('form_alter') as $module) {
    $function = $module .'_form_alter';
    $function('filefield_js', $form);  // <= LINE 852
  }
  $form = form_builder('filefield_js', $form);

  $output = theme('status_messages') . drupal_render($form);

  // Send the updated file attachments form.
  print drupal_to_js(array('status' => TRUE, 'data' => $output));
  exit();
}

The file uploads, but the error message is disturbing. I'm marking this as critical because I need to launch my site ASAP, but can't as long as this error message is appearing.

Please help?