The superstitious president - logic puzzle

Mr. Fisher is a night watchman in a large company. On a certain morning when Mr. Fisher wants to go home, his boss tells him: "I'll go for a businesstrip to Norway. Tomorrow I will depart from Heathrow airport." Mr. Fisher however advises him to take a boat. "Why should I?" asked the president. "This night I dreamed that the plane to Norway crashes, just before it will land," is the response. The president smiles first, but since he is pretty superstitious he decides to take the boat. When he arrives in Norway, he is told that the plane which he should have taken had crashed. When the president returns from the trip, he gives a big reward to Mr. Fisher and immediately fires him. Why?