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(CNN) —  

Special challenge coins are being distributed among US Secret Service personnel and their families, who are expressing frustration at the requirement they work without pay during the partial government shutdown.

The coins are paid for by agents or other personnel on their own – they are not official and use no government money.

“DON’T WORRY, YOU’LL GET BACKPAY,” is printed on one side of the coins, according to a photo obtained by CNN’s Jake Tapper. The other side has “UNITED STATES SECRET SERVICE” and “ESSENTIAL PERSONNEL” printed on it.

PHOTO: Obtained by CNN

Challenge coins are historically collectable pieces of recognition that originated in the military, but have recently become more popular with civilian agencies.

Secret Service agents told CNN last week that the lack of pay is hurting morale and affecting daily operations as they continue to carry out their mission without pay. Parts of the government have been shut down for more than a month, with no end in sight.

The Secret Service, which falls under the Department of Homeland Security, employs 7,222 people. Nearly 6,000 of them are working without pay and more than 1,200 have been furloughed since they were not deemed essential, according to the Department of Homeland Security’s shutdown plans.

The Secret Service has field offices around the world. It is responsible not only for protecting the President of the United States and his family but also embassies and foreign dignitaries within the US, in addition to its core mission of investigating financial crimes.

CNN has reached out to the Secret Service and the Department of Homeland Security for comment on the coins.

CNN’s Jake Tapper and Sara Ganim contributed to this report.