A Chipotle restaurant in Massachusetts has voluntarily closed for a cleaning because of norovirus concerns after at least one employee contracted the illness, according to the local news station WHDH.

"After learning that four of our employees were not feeling well, our restaurant in Billerica, Mass. was closed for a full sanitization," a Chipotle spokesperson told BuzzFeed News. "No customers illnesses are connected to this restaurant."

It is not clear when the Billerica restaurant will reopen.

The closure comes just a month after the restaurant chain closed all of its restaurants for a morning meeting to discuss new food safety protocols with its employees, which include testing for pathogens, new cooking methods, and changes with suppliers. The chain also emphasized the need for workers to stay home if they are ill.

A norovirus outbreak at a Chipotle restaurant in California last year prompted a criminal investigation, where federal prosecutors subpoenaed the company's food safety records. There was also a norovirus outbreak in Boston in December.

A series of norovirus, E. coli, and salmonella outbreaks in 2015 also caused the company's sales to drop sharply: Chipotle said its sales fell almost 15% in the last three months of 2015.

The company's stock fell 4% in after-hours trading on Tuesday.

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