WASHINGTON, April 11 (UPI) -- Those lucky enough to score tickets for papal masses next week in New York and Washington are being warned that selling them is a sin.

Ticket sellers would also be cheating the buyers, the Chicago Tribune reports. Admission to the masses requires not only a ticket but photo identification.

Pope Benedict XVI is to celebrate mass at Nationals Park in Washington Thursday and at Yankee Stadium in New York April 20. The Washington stadium holds 46,000 and Yankee Stadium 70,000.

"You cannot sell a sacrament," warns Susan Gibbs, communications director for the Roman Catholic archdiocese of Washington.

Gibbs said even selling a ticket to donate the proceeds to charity would be wrong.

The Tribune identified the sin as simony, named after Simon Magus, who, as described in Acts of the Apostles, offered two of Jesus's disciples money for transferring to him the power to fill people with the Holy Spirit.

At least one ticket to the New York mass is for sale on eBay. But buyers are warned on the site that all they will get is a souvenir, not admission to Yankee Stadium.