Twilio ($TWLO) is raising a flag to caution shareholders about a potential impact on future revenue. During the company’s first quarter earnings call, chief executive Jeff Lawson warned about a change with one of Twilio’s largest customers: Uber. He said that contributions from the on-demand ride hailing service has started to decrease, from 17 percent last quarter to 12 percent this quarter, and Twilio “expects contributions to decline further” — it expects the associated revenue to drop sequentially and year over year.

Lawson attributed the decline to Uber’s growth, suggesting that there’s a push to move some of the communication in-house. Uber currently uses Twilio for a variety of use cases, such as driver marketing and communication. Despite this, Twilio said that it believes “Uber will remain an important customer moving forward” while stating that it’s due to Uber and WhatsApp being outliers rather than a sign about Twilio’s stability.

Today’s news is an example of one of the risks the cloud-based telephony company warned against when it was going through the process of going public. When it filed in May 2016, Twilio disclosed that Facebook-owned WhatsApp accounted for 15 percent of its revenue for the first three months of 2016 with no long-term contract. Uber appeared to be the largest customer using the communication platform, accounting for 17 percent.

The usage reduction by Uber was attributed to the fact that, because of the company’s massive growth and reach worldwide, the service wound up being expensive, and thus Uber’s management made a business decision to reduce dependency on Twilio’s platform. “It’s the nature of the company and size of the spend for Uber to make this kind of decision,” Lawson said, while denying that this was part of a growing trend investors should be worried about.

He reiterated that besides WhatsApp and Uber, Twilio has “very little customer concentration that follows.” In fact, its third largest customer accounted for 2 percent of revenue.

Twilio chief financial officer Lee Kirkpatrick said that the company forecasted “modest growth” by Uber in 2017, but has since reduced its expectations “significantly.” It’s perhaps this adjustment that has resulted in Twilio updating its guidance, something that has caused shares in the company to tank after the release of its quarterly earnings.

While the call did focus on Uber, Kirkpatrick denied any changes coming around WhatsApp usage.