The Potters were awarded a spot kick when one down in their Premier League fixture this afternoon but Walters saw his penalty saved by the English stopper.
West Brom went on to win 2-1 and edge further away from the relegation zone.
"We had a actually done a bit of homework," said Foster. "Dean Kiely, the goalkeeping coach, showed me the penalties before the game and I knew if Jonathan stepped up then I was always going to go to the right."
Roy Hodgson's men are now seven points clear of the drop zone.