Just five weeks after Clown Shoes Beer began shipping bottles of Archdruid, the Ipswich, Mass.-based company has issued a statement confirming that an unknown number of bottles are infected.

The company is asking anyone with bottles they think might be infected to send in labels or receipts, though it has not stated whether it will issue refunds.

According to a post on the brewery’s website, “a small amount of lactobacillus found its way into an unknown portion of the bottled product” causing “varying degrees of souring of the product, ranging from slightly tart to medium sour.” The beer is a 9 percent ABV imperial Irish red was aged in Irish whiskey barrels for three months.

The address to send information to can be found here.

“The quality control processes we employ have worked to date, but we are aggressively looking into even more sophisticated ways to minimize issues in the future,” reads the statement on Clown Shoes’ website. “It would be beyond fantastic if there was a failsafe to prevent contamination of barrel aged products.  But there isn’t.”

An email sent to the brewery for clarification on the purpose behind sending in receipts and labels was not immediately returned.