Is expiation obligatory for broken oaths if the enquirer did not know beforehand that the expiation is feeding ten poor people and did not make allowances for that?

Whoever swears by Allah to do something and then breaks his oath, an expiation is incumbent upon him. This expiation is: feeding ten poor people from the average of what you feed your own families, or clothing them, or freeing a slave. If one cannot find [any of these], then it is fasting three days. It is not a condition that the one who swore be aware of the expiation.