graythresh dosn't work with variables other than uint8?

Say I have a matrix B (some image):

B=[1 2 3 ;5 6 7 ; 10 11 12;14 56 45];

if the B is defined as uint8, graythresh returns a level:

level=graythresh(B)

ans =

    0.1137

But, if B is defined as a uint16 or double (which the documentation says it supports), it returns 0.

B= uint16(B)

B =

4×3 uint16 matrix

1    2    3
5    6    7
10   11   12
14   56   45

level=graythresh(B)

level =

     0

What is going on? Thank you.