Ahead of early holiday shopping, United Parcel Service Inc. says it has been warning UPS Freight customers of potential delays. On Monday, Nov. 12, some 11,600 UPS Freight workers, who are also members of the International Brotherhood of Teamsters union, may strike.

UPS posted a warning on its website and confirmed to Fortune in a statement that the company has already taken precautions against shipping delays by ceasing to accept new pickups and emptying its network of freight by Friday, Nov. 9. UPS Freight picked up new heavy shipments until Wednesday, Nov. 7, and only guaranteed delivery of ground freight shipments through Thursday, Nov. 8.

“The company has now begun discussions with UPS Freight customers to inform them of the potential for service disruption and the need to arrange alternative carriers. Because we do not have a guarantee against a work stoppage, we cannot afford to put our customers’ volume at risk of being stranded in our system,” UPS said in a statement.

In October, the Teamsters union ratified a five-year labor agreement for UPS employees. But a separate deal specific to UPS Freight workers was rejected and couldn’t be ratified. The new vote is scheduled to be counted on Sunday, Nov. 11. If unionized workers vote to ratify the contract, UPS confirmed that the company will begin accepting new volume from UPS Freight customers.

Freight is a smaller UPS division than the company’s small-package division, which employs more than 243,000 sorters and truck drivers, among other workers.

The last major UPS shutdown was more than two decades ago, when contract negotiations broke down and 186,000 workers went on strike for 16 days in 1997. At the time, it was one of the biggest nationwide strikes ever, and it cost UPS hundreds of millions in losses.

In the same statement, UPS noted, “We are disappointed that the Freight Teamsters union leaders have chosen to announce the potential for a strike, should their members vote ‘no’ on the offer. A vote is scheduled to be counted on Sunday, November 11. With a ‘yes’ vote, we will resume operations and begin accepting new volume from UPS Freight customers as soon as scheduled pick-up and delivery orders are confirmed.”

Up for further debate are issues such as wage increases and worker paid time off, including personal days and vacation.

In a statement, the Teamsters union wrote, “This could all have been resolved by the company [UPS] addressing the key issues at the negotiating table.”