The Mishna on Nedarim 18b says that if a person made an oath to idolatry - it is not a valid oath; if "Stam" - it is valid.
Rashi's first explanation is that the oath was indeed not explicit, and when the person came to ask whether he must keep the oath he explains that he meant the oath to be to idolatry - he need not keep the oath.
Why do we believe him? "Ein Adam Oseh Atsmo Rasha"?
The principle of "Ein Adam Mesim Atzmo Rasha" applies only to Edus (see Rashi Yevamos 25b).