ceph_crc32 null buffer

Hi all, Does anyone know that why Ceph needs to calcualate the crc of "null" buffer? There's below code in Ceph: ceph_crc32c(rand(), nullptr, size); https://github.com/ceph/ceph/blob/master/src/test/common/test_crc32c.cc#L358 For the "null" buffer case, doesn't Ceph think to be all zero value to calculate the crc value? B.R. Changcheng