The eagerly anticipated final film in the vampire saga, Breaking Dawn - Part 2, is due out next month and it is already known that the conclusion will be different to the ending in author Stephenie Meyer's book.
R-Patz told HollywoodLife.com: "It's very strange the ending but it kind of works in a very bizarre way.
"You don't normally get a story where everyone has a happy ending. Which sounds kind of ridiculous but even if you tried to write a happy ending it's kind of impossible."
But he revealed writer Stephenie had changed the storylines, "to make everyone have a happy ending".
"Which creates this very odd drama at the end," Robert revealed.
The British star - who is gearing up to promote the flick with co-star and real-life girlfriend Kristen Stewart - said the ending of the final instalment in the blood-sucking franchise is "probably the sweetest moment in the whole film".
"It's good," he added.