Error with USPS shipping

I have enabled the USPS plugin and contacted the postal service to switch my account over to the production server. However, I am not seeing shipping being calculated for an order and am seeing the following two errors in my log when getting to the confirm-checkout step:

[Fri Apr 20 10:55:38 2012] [error] [client <ip_removed>] PHP Warning: Invalid argument supplied for foreach() in /var/www/html/wp-content/plugins/marketpress/marketpress-includes/plugins-shipping/mp-usps.php on line 492, referer: https://<host_removed>/store/cart/checkout/
[Fri Apr 20 10:55:38 2012] [error] [client <ip_removed>] PHP Warning: Invalid argument supplied for foreach() in /var/www/html/wp-content/plugins/marketpress/marketpress-includes/plugins-shipping/mp-usps.php on line 793, referer: https://<host_removed>/store/cart/checkout/

Any idea what might be causing this?

Thanks.