EINVAL on sendmsg() to a UNIX Doman Socket

I'm seeing an EINVAL result when trying to do a sndmsg() call to a UNIX socket. The man page says that means that the sum of the iov_lens overflows an ssize_t, but an ssize_t is 8 bytes on this machine and there's only one iov and its length is 671. Last I checked that doesn't overflow anything but a char. What gives? Same code works fine in Linux and when using UDP or TCP.