It’s cruel to administer this oath to a man and then withhold from him the power to carry out that oath. Especially if fulfilling the oath is presented as his divinely ordained duty on which his salvation depends.
Pope Zachary said something similar to Pepin the Short.
“Restore the things that are gone to decay, maintain the things that are restored, punish and reform what is amiss, and confirm what is in good order: that doing these things you may be glorious in all virtue; and so faithfully serve our Lord Jesus Christ in this life.”