The one genre rule

It was a normal day and I decided to read one of my older books. I then noticed that Adventure in a fantasy land(which is supposed to be the sequel of The Music book) has a totally different genre than the first book and has nothing in common with the first book. Here is my compare and contrast
Fantasy land: A fantasy/ sci fi
Music book: A realistic fiction(well not that realistic) and takes place in normal times.
Both: same characters
I then knew that when you are writing sequels that you should try to make the books the same genre. You know if you are writing a fantasy book, try to make the sequel a fantasy. This rule doesn't apply with everything. You can have the first book being an action novel and the sequel be more of a romance. But just don't make it too romancy, add some sprinkles of action(or the original genre) into the book.
The one genre rule was yet another thing I learned from my old books. See you gotta learn from your mistakes.
-AM