(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.

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.

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