You had one job, McDonald’s!

When the fast-food giant announced it was celebrating Grimace’s birthday with a limited-edition purple shake, I was excited.

Could it possibly be an Ube-flavored shake? The delicious and nutty purple sweet potato from the Philippines and my youth has become the “it” ingredient in foodie culture of late, popping up in everything from cheesecakes and ice cream to cookies and waffles.

But after finally trying Grimace’s Birthday Shake, I can report it’s definitely not Ube. What’s its deal then?

The backstory: Grimace made his debut in 1971, appearing in an ad campaign along with other McDonaldland characters such as Ronald McDonald, Hamburglar and Captain Crook. The big, fuzzy blob originally had four arms, which he used to steal milkshakes from customers. In fact, the purple bestie loves milkshakes so much, he celebrates every birthday at McDonald’s. So, in honor of the character’s 52nd trip around the sun, the chain introduced the Grimace Birthday Meal on June 12. It comes with a choice of a Big Mac or 10-piece Chicken McNuggets, plus fries and that purple, not-Ube shake.

What you need to know: Knowing a social media opportunity when it sees one, McDonald’s has been intentionally coy about the shake’s flavor. The creamy concoction is “inspired by Grimace’s iconic color and sweetness,” the company said in a press release. And the menu item’s nutritional info page is of no help, indicating it’s made with vanilla reduced-fat ice cream and “Grimace Shake Syrup.”

Scrolling down, there’s nothing in the ingredients of Grimace Shake Syrup that provide any clues of the intended flavor either. It’s just the usual sugar, water, corn syrup and the ominous “natural flavor.” A company spokesperson, however, recently told the “Today” show the sweet treat contains “berry flavors.”

OK, but I wanted to be sure.

The taste test: Fast-food shakes can often be too thick or too runny. But this one was right in that sweet spot of moderately creamy. The first flavor that popped into my mind was grape jelly, but that was probably just the purple talking. I took a second and third sip and noted how sweet the shake is. So, I mixed the whipped cream on top and the vanilla ice cream at the bottom into the purple midsection and that balanced it out nicely. I then tried to narrow in on the flavor with each subsequent sip. Birthday cake? Maybe. Cereal milk? Perhaps. But it’s more like if someone dropped a Wild Berry Pop Tart into a fairly standard vanilla milkshake and then hit the pulverize button on the blender. Nice, as Dom DeLuise in “History of the World Part I” might say. Not thrilling, but nice.

If you go: Grimace’s Birthday with Special Meal & Shake is available at participating McDonald’s restaurants nationwide while supplies last via the app, in-restaurant or delivery. I paid $12.79 for my Big Mac Meal & Shake, while the McNuggets option costs $13.49. Prices may vary by location.