why set tcp check CSUM_MANGLED_0?

all: I am reading tcp checksum code. uh->check = csum_tcpudp_magic(src, dst, len, IPPROTO_UDP, csum); if (uh->check == 0) uh->check = CSUM_MANGLED_0; #define CSUM_MANGLED_0 ((__force __sum16)0xffff) uh->check will be 0, but will never be 0xFFFF. Why set uh->check to CSUM_MANGLED_0?