Benson scored a late equaliser to make it 3-3 but was then shown a red card two minutes from time for a foul on John White.
Manager Phil Parkinson was not convinced by the referee's decision, and said: "I'm a bit frustrated, it was a yellow card at the absolute most."
The Addicks will be without striker Akpo Sodje (shoulder) for another two weeks, while forward Joe Anyinsah was left out of the squad for the Us clash as he continues to be plagued by spinal injuries.
Leading scorer Johnnie Jackson made it three penalties in two league games against his former club to bring his tally to 10 for the season.
Source: PA
Source: PA