Why does Doc think Jane Wyman is First Lady?

When Marty goes to Doc and tries to explain that he is from the future, Doc initially doesn't believe him. And when asked who is president in 1985, and Marty says Ronald Reagan, Doc is in disbelief saying "Ronald Reagan? The actor? A bit later, as Doc is heading to his garage, he says "I suppose Jane Wyman is First Lady". But why would Doc think Jane Wyman is First Lady? As by 1955, Ronald Reagan was already married to Nancy.