Monster Beverage Corp. (MNST) shares plummeted 12.98% in pre-market trading after the Corona, California-based company revealed that its business partner and largest shareholder, Coca-Cola Co. (KO), is planning to launch two energy drinks of its own.

The two firms struck a deal in 2015 that restricted Coca-Cola, owner of a roughly 18% stake in Monster, from competing in the energy drink market. However, that agreement also contained “certain exceptions.” Coca-Cola is now seeking to market its new products under an exception, but Monster believes it does not apply in this case.

In an earnings call, Monster CEO Rodney Sacks said the dispute entered into arbitration last week as both companies seek to determine their respective rights. Monster’s CEO added that the disagreement led Coca-Cola to delay the launch of its potential new energy drinks until April 2019.

“There is an issue in an agreement, which we've agreed to go to arbitration civilly and determine what course of action is appropriate,” Sacks told analysts.

He was eager to reassure investors that Monster’s relationship with Coca-Cola has not been damaged by these developments. “Nothing has changed in the relationship and the manner in which this situation will be dealt with will be conducted from both parties in — on a civil basis according to the agreement,” he said. “We don't believe it will have a majority impact on our relationship. We just believe we have to manage it in an appropriate way if and when it occurs.”

News that Coca-Cola could be planning to enter Monster’s turf took the shine off the Corona, California-based company’s encouraging quarterly earnings. Prior to discussing the disagreement, Monster revealed that third-quarter earnings jumped 26% to 50 cents per share as sales rose nearly 12% to $1.02 billion, marking only the second time that the energy drinks giant has posted quarterly revenues over $1 billion.

Evidence that the company is prospering in a difficult beverage industry initially saw the shares climb 6%.