Mscohere-why would the coherence tends to 1 as nfft increases?

Dear all,

Does anyone have a clue as to why -using mscohere- the coherence becomes exactly one for all frequencies if nfft is set to about 190-200. I know it's standard practice to set it to a number that is a power of 2. I tried with 1024, and all I got was a bunch of NaN. So I started with low numbers, and the coherence seem to have plausible numbers until I hit the 190 mark. I'm completely baffled.

Looking forward to hearing from you.