(let ((b (make-array 1 :element-type '(unsigned-byte 8))) (m (make-array 1 :element-type 'fixnum :initial-element 3))) (setf (aref b 0) (logxor (aref m 0) (aref m 0))) b) returns #(3), while it should return #(0) -- With best regards, Stas.