FMC message: NAV INVALID - Why?

Hi,Sometimes, usually when I change my pre-selected approach on the DEP/ARR page in the FMC and press EXEC, the message "NAV INVALID" appears.This also happens to me when taking off from KSEA, circling around and trying to choose a runway for landing on the DEP/ARR page. I always have to land manually with no navigation, not that it's a problem to land manually in fair weather, but I'd like to know why this FMC message appears.Is this a bug? If not, what am I doing wrong?Edit: I don't have this problem when flying a programmed route from one airport to the other without changing the approach/runway.Best regards,