Missing A key signature

I was working on an original score, and went to change my key signature to A major . . . only to find there is no A major key signature available in the palate. All of the other keys are there--just A is missing. To be honest, I don't know what else to say . . . it's just gone.
How can I fix this?
For the record, I'm using Musescore 3 on Windows 10. Don't know if that makes a difference, but I thought it might help.