WhatsApp founder Jan Koum WSJ/Screenshot Today, hot communications startup Twilio filed to go public, which also means that we're getting our first serious look into its financials.

You can read the filing here.

Customers like Uber and Nordstrom rely on Twilio's services to automatically send text messages and phone calls.

But per Twilio's filing today, Facebook's WhatsApp messaging app is the company's single biggest customer, and has been since 2013. In 2015, WhatsApp accounted for a solid 17% of the $166 million in revenue generated by the company.

With almost 1 billion users, and no sign of slowing down, WhatsApp is a great customer for Twilio, especially as Twilio looks towards an IPO. The bigger WhatsApp gets, Twilio says, the more use of the service they can claim.

But there's a big catch: WhatsApp and Twilio have absolutely no long-term contract, warns Twilio's filing today.

"WhatsApp has no obligation to provide any notice to us if they elect to stop using our products entirely and, as such, the contribution from WhatsApp could decline to zero in any future period without advance notice," says the filing.

That's doesn't inspire a lot of confidence: While Twilio also says in the filing that it's expecting WhatsApp to account for less overall revenue as it attracts more and larger customers, the company isn't profitable today, and the prospect of losing as much as 17% of overall revenue could spook investors.

So in some real ways, it's up to Facebook and WhatsApp to decide whether or not Twilio will thrive on the public markets. If they ever change providers, or build their own Twilio replacement, that would seriously damage the company's prospects.