What: Shares of Cardtronics (NASDAQ:CATM), the largest non-bank ATM operator in the world, slumped on Tuesday after its largest customer, 7-Eleven, did not renew its ATM placement agreement. After falling as much as 16% at market open, the stock was down 10% at 11 a.m.

So what: 7-Eleven has been a Cardtronics customer since 2007, and the current ATM placement agreement between the companies expires in mid-2017. Once the agreement expires, 7-Eleven will switch to an ATM provider that is a related entity to its parent company.

7-Eleven is Cardtronics' largest merchant customer by far, accounting for 17.5% of revenue in 2014. Cardtronics doesn't expect the news to alter its 2015 financial guidance, which it issued in April of this year.

Now what: Cardtronics has been growing at a healthy pace, increasing revenue by about 20% in 2014, so while losing 7-Eleven as a customer will be a major setback, it's not the end of the world for the company. Cardtronics CEO Steve Rathgaber had this to say: "We are proud of the service and unique products that we have delivered to 7-Eleven since 2007. While we are disappointed in this decision, we have every confidence in our business and the robust growth opportunities ahead of us."

Investors are certainly justified in punishing the stock, but by the time the 7-Eleven deal officially ends, Cardtronics will have likely replaced that lost revenue through growth. The loss of 7-Eleven appears to be a setback, albeit a major one, and not a deeper problem with the company.