The police union in St. Paul, Minnesota is suing the city in an effort to halt a mandate that requires city employees to get vaccinated against COVID-19 by the end of the year.

St. Paul Mayor Melvin Carter announced the requirement for nearly 4,000 workers last month, the St. Paul Pioneer Press reported. But the St. Paul Police Federation has resisted, suggesting that unvaccinated employees be allowed to continue working if they wear masks and take regular tests for the virus. It filed the lawsuit Tuesday in the Minnesota court system.

Police departments around the U.S. are running up against pockets of vaccine resistance that some fear could leave law enforcement shorthanded and undermine public safety.

The St. Paul police union’s lawsuit argues that the vaccine requirement amounts to a new condition for employment that was not negotiated with the union. It is asking a judge to halt the mandate until the union and city negotiate an agreement.

“We are not anti-vaccine, nor are we conspiracy theorists — we are reasonable and dedicated public servants who believe in personal choice,” the union said in a statement.

The mayor’s office said it planned to move forward with the mandate unless a judge orders to halts it.