I get this error after I install MarketPress and activate

Hello!

I get this error after I install MarketPress and activate global cart with Paypal Express Checkout.

Why is that and what can I do?

Fatal error: Cannot use string offset as an array in /home/playxpb/public_html/bazaarbee/wp-content/plugins/marketpress/marketpress-includes/marketpress-ms.php on line 220