Causes 500 Internal Server Error after saving a page and clicking 'view page' in the updated notice.

gistfile1.php

<?php

/**
* add discount to logged in users based on available capabilities
*
* @param mixed $cart
*/
function vpm_apply_discount( $cart )
{
    if( ! is_user_logged_in() ){
        $cart->remove_coupons();
        return;
    }

    if( current_user_can( 'view_gold' ) ){
        if( ! $cart->has_discount( 'VPMGOLD20' ) ){
            $cart->add_discount( 'VPMGOLD20' );
        }
    }

    if( current_user_can( 'view_silver' ) ){
        if( ! $cart->has_discount( 'VPMSILVER15' ) ){
            $cart->add_discount( 'VPMSILVER15' );
        }
    }

    if( current_user_can( 'view_bronze' ) ){
        if( ! $cart->has_discount( 'VPMBRONZE10' ) ){
            $cart->add_discount( 'VPMBRONZE10' );
        }
    }
}
add_action('woocommerce_before_calculate_totals', 'vpm_apply_discount' );