This summer, you haven't truly lived unless you've Instagrammed yourself holding a Starbucks "rainbow drink." These concoctions are custom creations that are just as tasty as they are pretty, and come in all colors of the rainbow.

These popular creations, though not officially anywhere on the Starbucks menu, usually come crammed with one key ingredient: coconut milk. And that just so happens to be a main ingredient in Starbucks' newest drink, the iced coconut milk mocha macchiato. Adding coconut milk makes the drink "lighter in body" and "more refreshing," a spokesperson said when the drink was released July 11.

But all the love for coconut milk might be straining the supply at your local Starbucks. Business Insider reports that customers are complaining that their favorite locations are running out of coconut milk like crazy, leaving them without the treats they showed up for. It's also frustrating for people who prefer non-dairy substitutes in their coffees in general.

On a Reddit section dedicated to Starbucks, a poster claiming to be an employee said their store was "out of coconut milk for the past few days and people are pissed," especially if they put in a mobile order that included coconut milk. Customers are venting their frustration on social media:

Starbucks says that their supply of coconut milk is just fine in general, though specific stores might run out from time to time, just like they might run out of any other ingredient. "We're happy to confirm that Starbucks has a healthy coconut milk supply," a spokesperson for Starbucks told Business Insider. "As you can imagine, our stores may run low on product from time to time, but this is not something we're seeing at our stores across the board."