In /includes/classes/shopping_cart.php (from v1.5.2, RC2), the function actionAddProduct checks for an undefined array index ([$i]) within the cart_quantity POST variable. I'm pretty sure that removing the additional index will correct the issue.
Code:
  /**
   * Method to handle cart Action - add product
   *
   * @param string forward destination
   * @param url parameters
   */
  function actionAddProduct($goto, $parameters) {
    global $db, $messageStack;
    if ($this->display_debug_messages) $messageStack->add_session('header', 'A: FUNCTION ' . __FUNCTION__, 'caution');

    if (isset($_POST['products_id']) && is_numeric($_POST['products_id'])) {
      // verify attributes and quantity first
      if ($this->display_debug_messages) $messageStack->add_session('header', 'A2: FUNCTION ' . __FUNCTION__, 'caution');
      $the_list = '';
      $adjust_max= 'false';
      if (isset($_POST['id'])) {
        foreach ($_POST['id'] as $key => $value) {
          $check = zen_get_attributes_valid($_POST['products_id'], $key, $value);
          if ($check == false) {
            $the_list .= TEXT_ERROR_OPTION_FOR . '<span class="alertBlack">' . zen_options_name($key) . '</span>' . TEXT_INVALID_SELECTION . '<span class="alertBlack">' . (zen_values_name($value) == 'TEXT' ? TEXT_INVALID_USER_INPUT : zen_values_name($value)) . '</span>' . '<br />';
          }
        }
      }
      if (!is_numeric($_POST['cart_quantity'][$i]) || $_POST['cart_quantity'][$i] < 0) {
        // adjust quantity when not a value
        $chk_link = '<a href="' . zen_href_link(zen_get_info_page($_POST['products_id']), 'cPath=' . (zen_get_generated_category_path_rev(zen_get_products_category_id($_POST['products_id']))) . '&products_id=' . $_POST['products_id']) . '">' . zen_get_products_name($_POST['products_id']) . '</a>';
        $messageStack->add_session('header', ERROR_CORRECTIONS_HEADING . ERROR_PRODUCT_QUANTITY_UNITS_SHOPPING_CART . $chk_link . ' ' . PRODUCTS_ORDER_QTY_TEXT . zen_output_string_protected($_POST['cart_quantity'][$i]), 'caution');
        $_POST['cart_quantity'][$i] = 0;
      }