WeldWerks Brewing Co. is offering refunds on one of its most popular releases after signs of infection were found.

The affected beer is Barrel Aged Mexican Achromatic—otherwise known as BAMA—a 13.2 percent ABV imperial stout brewed with “obscene amounts” of roasted cacao nibs, cinnamon sticks, and vanilla beans before being aged in Breckenridge Distillery bourbon barrels. According to a post on the Greeley, Colo.-based brewery’s website, a number of the beers have developed “off flavors” consistent with souring bacteria, and although just how widespread the issue is remains unknown, all of the BAMA bottles were filled in one batch so the issues could possibly be affecting any of the bottles that were released.

“After extensive testing and lab analysis performed over the past several weeks, we have discovered that some bottles of Barrel Aged Mexican Achromatic have developed off-flavors, most likely caused by souring bacteria,” reads a statement on the brewery’s website. “These off-flavors are most commonly characterized by a noticeable tartness or acidity, reminiscent of sour cherries. We do not know exactly how widespread the issue is, nor how many bottles are affected and as a result, we are recommending that any remaining bottles be stored cold and consumed sooner rather than later.”

News of the issues with Barrel Aged Mexican Achromatic comes just six months after Goose Island Beer Co. offered refunds after signs of infection were found in four out of the six Bourbon County Brand Stout variants that were released in 2015: Bourbon County Bourbon County Brand Stout, Bourbon County Proprietor’s Stout, Bourbon County Brand Coffee Stout and Bourbon County Brand Barleywine.

WeldWerks is offering to refund the purchase price of $14 via check for any anyone who has experienced issues with any of their bottles. Customers who have affected bottles must complete the following process in order to apply for their refund:

Write your first initial and last name in permanent marker or pen on the left side of the label, along with the current date. If requesting refunds for multiple bottles, simply add an alphabetical suffix to the end of the date, starting with “A”, then “B”, etc.

Take a picture of the bottle, clearly showing your first initial, last name, and date on the label, as well as the top of the bottle with the cap removed.

Email the picture as an attachment, along with your full name, address and phone number, to [email protected] before May 1, 2017.

According the post, WeldWerks has already started the process of “implementing new quality control protocols to avoid similar issues in the future.”