Monitor - error 6 pregnancy test

Hello, I have the fertility monitor and have been mostly satisfied so far. This month my period is late and I did a pregnancy test yesterday and today. Both times it came back with no result and instead the same error number 6, unfortunately I can't find that error in the manual. What does it mean? I've been successfully testing for a while now so I don't think I made a mistake using the test sticks. Thank you!