Anybody here know of a 16-bit CRC type thing that doesn't output 0 or 0xFFFF as a valid result?

This is to store checksums in flash and I need to be able to detect 0xFFFF for 'no checksum written here', 0 for 'checksum was erased' and any other value for 'this is a valid checksum'.

I could take a `(crc_u16 % 0xFFFD) + 1`, but then number value 1 and 2 have double the collision chance.
Maybe that's fine, idk...