Amazon.com has been accused by a U.S. labor board of illegally refusing to bargain with a union representing drivers employed by a contractor, the agency announced on Wednesday.

The complaint from the National Labor Relations Board claims that Amazon is a so-called “joint employer” of drivers employed by the contractor, Battle Tested Strategies (BTS), and used a series of illegal tactics to discourage union activities at a facility in Palmdale, California.

BTS drivers voted to join the International Brotherhood of Teamsters union last year, becoming the first Amazon delivery contractors to unionize.

The NLRB in the complaint, which was issued on Monday, said Amazon broke the law by terminating its contract with BTS after the drivers unionized without first bargaining with the Teamsters.