4.02 buffer wants char, not int, for some reason

Error: The implementation sha1.ml does not match the interface sha1.cmi:
       Type declarations do not match:
         type buf =
             (int, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
             Bigarray.Array1.t
       is not included in
         type buf =
             (char, Bigarray.int8_unsigned_elt, Bigarray.c_layout)
             Bigarray.Array1.t
       File "sha1.ml", line 17, characters 5-81: Actual declaration