The umpires originally ruled it a double but after reviewing it, they awarded Walker a home run to give the Pirates an 11-6 lead.
"I didn't think it was a home run," Walker said. "The kid at first base [Brandon Belt] thought it might have hit the chair and popped back out, but I thought it hit the wall, just because of the way it bounced back to him."
It was Walker's fifth home run of the season and third RBI of the night.
Steven Petrella is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.