You are what you write

Here is something that I have been having a hard time with. Some people say that you should write what you know leaving less room for error. I, however, disagree. I believe that every writer should expand into unfamiliar territory. I have done this myself, with hilarious results. Of course I showed those who are dearest to me (they laughed too), but it's all about taking the risk. Maybe you have a knack at Non-fiction, but haven't tried because you are so wrapped up in your novel you have been working on for 5 years. I just wanted to hear thoughts from others.