When I first experienced this I ignored it. But after some days I noticed that I had to unlock it by pattern lock or password everytime I restart the phone and after unlocking by pattern/password, I was able to use fingerprint.
Why is this so? I am not sure that every Android device has the same issue. Is this a security feature? If yes, why? I mean if this is a security feature the phone must be unlocked by fingerprint, as pattern or password can be guessed after some attempts (especially patterns).