Commentary Purg XX 53-54

Again Dante is misled, perhaps confusing events in the eighth century surrounding the last days of the Merovingian line, when Pippin the Short did put away his last possible political rival in a monastery.  What Hugh had done was to have the last of the Carolingians, the Duke of Lorraine, imprisoned in 991.  He remained in prison until his death a year later.  See Bosco/Reggio (DDP Bosco.Purg.XX.54).