The Lumberjacks took over at the ISU 18 after Robinson's first fumble, but the Redbirds turned the visitors away with a Charles Woods interception.
Northern Arizona missed a 51-yard field goal attempt after the early third-quarter miscue.
"It's not that we want the ball to get turned over, but our defense thrives off those negative situations," linebacker Ty DeForest said. "We love nothing more than being put on a tight field and go out and do what we do.
Offensive explosion: The ISU offense totaled 534 yards and its 419 passing yards were more than the 3-1 Redbirds had over the first three games combined.