Free throw violation by defense

[donj]

Two shot foul. Official administered ball to shooter for first free throw, backs out and then notices one of bottom spaces not occupied by defense. He signals for a free throw lane violation by defense and free throw subsuqently is missed. He points out violation and allows shooter to shoot first shot over. Initially, the official erred by not insuring lane space occupied by the defense, but was this the correct procedure given the circumstance? Please give rule and/or case # for this situation for future reference. Thanks