Cosi, the 109-unit sandwich and salad chain, didn’t blame the weather, the economy or shifting trends for its weak third quarter same-store sales.

The Boston-based chain blamed Pope Francis.

“Everyone was hoping it would be a great thing for business,” Chief Executive R.J. Dourney said of the pontiff’s visit to the US last month, “but it ended up being more of a challenge.”

The chain’s same-store sales declined 0.3 percent in the quarter. About 30 percent of Cosi’s restaurants are located in the three cities the pope visited: Washington, DC, Philadelphia and New York City.

In the wake of the news, Cosi shares fell 17 percent on Wednesday, to 89 cents.

Dourney was appointed chief executive last March to spearhead a turnaround of the 19-year-old company, which closed 10 stores last year. There are 79 company-owned eateries and 30 franchises in 15 states.

Known for its hearth-cooked pocket bread, Cosi is getting a $2.5 million makeover, Dourney said.