This question already has an answer here:
I've been sight reading through a book, "easy piano classics", part of the Bastien older beginner piano library. In a few of the arrangements, the key signature indicates that there will be sharps or flats, but those notes aren't used. For example, there is an arrangement in g major but it never contains the F#.
So why not just think of it as a mode of c major and keep the sharps out of it?