An important Walgreen revenue measurement slid more than analysts expected in December, even though prescription counts continued to recover for the nation’s largest drugstore chain since the resolution of a contract dispute.

The Deerfield, Illinois, company said Friday revenue from stores open at least a year dropped 6.1 percent last month as a shift toward generic drugs and a calendar quirk hurt the company’s sales. Pharmacy revenue tumbled 8.9 percent, while revenue from the front end, or rest of the store fell, 2.3 percent.