The Blues keeper, who was making his third appearance for the Sky Blues since moving to Coventry on an initial one month loan deal on 16th February, was dismissed for a deliberate handball outside the penalty area four minutes before half time at Highfield Road.
Sky Blues boss Micky Adams had no goalkeeper on the bench and so midfielder Stephen Hughes took over between the sticks and Coventry held out for a 0-0 draw against the Potters.
Benno now faces an automatic one game suspension for the red card offence and will miss Coventry's next game against Watford at Vicarage Road next weekend.