Krispy Kreme is celebrating “World Kindness Day” on Monday by giving out an extra sweet promotion. But there’s a catch.

The chain is giving away a box of a dozen glazed donuts for free with no purchase necessary. But only the first 500 guests that visit each participating Krispy Kreme store on Monday will be able to walk away with a free box of donuts. The promotion is taking place only in the United States.

Krispy Kreme often gives away free or deeply discounted donuts to generate buzz on special occasions. Earlier this year, it gave free donuts to high school and college graduates. In July, a dozen of glazed donuts were sold for 86 cents to celebrate its 86th birthday. And it routinely gives out free donuts to all customers (not just the first 500) on National Donut Day.

Promotions help to drive conversation around a brand and increase awareness. Free giveaways also can drive sales, because some customers who normally wouldn’t go to the store decide to make the trip and then make additional purchases, such as a cup of coffee.

Limited-time deals and scarcity deals like the first-come-first-served donut box giveaway can drive even more interest. But they also can risk alienating customers who make the trip only to find that they’re all sold out.

Krispy Kreme said it wants the brand associated with World Kindness Day to make “meaningful connections” with customers.

“World Kindness Day is an opportunity to make a positive difference by being generous,” Dave Skena, Krispy Kreme’s global chief brand officer, said in a release. “Simple gestures of caring and thanks, including sharing a sweet treat, is a great way to do that.”

The company also released earnings earlier this week, reporting that its revenue missed analyst expectations. Krispy Kreme said that it’s considering expanding a limited partnership it has with McDonald’s to sell more of its donuts at the latter’s location.

Krispy Kreme (DNUT) stock is up 20% for the year.