Steve Howard's fourth goal of the season had put the visitors in front after 55 minutes when Lloyd Dyer's cross went over the top of the defence and Howard came from around the back to place a powerful header past goalkeeper Mark Crossley.
Leicester charged forward again in the 38th minute and top scorer Matty Fryatt beat Sean Gregan on the edge of the area only to shoot wide when veteran Crossley cleverly narrowed the angle.
The Foxes came closest to breaking the first-half deadlock three minutes before the interval.
A shot from Howard hit a post and bounced straight to an unmarked Dyer but he skied the ball over the top from eight yards.
Then midfielder Whitaker grabbed his dramatic leveller when he floated a well-struck shot past goalkeeper David Martin from 25 yards.