Wishful Thinking

A reasoner who suggests that a claim is true, or false, merely because he or she strongly hopes it is, is committing the fallacy of wishful thinking. Wishing something is true is not a relevant reason for claiming that it is actually true.

Example:

There's got to be an error here in the history book. It says Thomas Jefferson had slaves. He was our best president, and a good president would never do such a thing. That would be awful.