[Plugin: Woocommerce] Fatal error: Call to undefined method

Hi,

im was searching for a way to add a custom Checkbox on the checkout Page. Finaly i found this:

// Agree Terms at Checkout

add_action('woocommerce_after_order_notes', 'my_custom_checkout_field');

function my_custom_checkout_field( $checkout ) {

    echo '<div id="my-new-field"><h3>'.__('My Checkbox: ').'</h3>';

    woocommerce_form_field( 'my_checkbox', array(
        'type'          => 'checkbox',
        'class'         => array('input-checkbox'),
        'label'         => __('I have read and agreed.'),
        'required'  => true,
        ), $checkout->get_value( 'my_checkbox' ));

    echo '</div>';
}

add_action('woocommerce_checkout_process', 'my_custom_checkout_field_process');

function my_custom_checkout_field_process() {
    global $woocommerce;

    // Check if set, if its not set add an error.
    if (!$_POST['my_checkbox'])
         $woocommerce->add_error( __('Please agree to my checkbox.') );
}

add_action('woocommerce_checkout_update_order_meta', 'my_custom_checkout_field_update_order_meta');

function my_custom_checkout_field_update_order_meta( $order_id ) {
    if ($_POST['my_checkbox']) update_post_meta( $order_id, 'My Checkbox', esc_attr($_POST['my_checkbox']));
}

I looks nice…the only Problem is:
If the checkbox is not clicked a Error Message should appear….instead this Message appears:

“Fatal error: Call to undefined method WooCommerce::add_error()”

Any Ideas?? Thank you 🙂