Contract Discharge by Objective Impossibility

Paulo, an importer of nuts and dried fruits, travels internationally to source unique products for his retail business. During a visit to Zulalania, he discovers a new variety of nuts and immediately contracts for a large shipment to the United States. Upon returning, he is notified by United States Customs that importing nuts from Zulalania is prohibited under current customs regulations. On what legal grounds can Paulo seek to be discharged from the contract?