Why mtx_sleep returning EWOUNDBLOCK?

Hi all,

Summary: I am running mtx_sleep() but getting an EWOUNDBLOCK response 
code and do not understand why.

I am looking at port code from the OpenBSD side to FreeBSD. I am 
assuming that tsleep_nsec()'s equivalent on FreeBSD is mtx_sleep(). To 
that end, I am running mtx_sleep as this:

mtx_lock(&(sc)->sc_mtx);
error = mtx_sleep(sc, &sc->sc_mtx, 0 , "athnfw", hz);
mtx_unlock(&(sc)->sc_mtx);

However, the error code returns a EWOUNDBLOCK. I would expect this to be 
0. From reading the man page for sleep(9), this means "A non-zero 
timeout was specified and the timeout". But if I slept for 1 hz, isn't 
that exactly what I want and thus it should return 0 (no error)?

The OpenBSD line in question is:

error = tsleep_nsec(&usc->wait_msg_id, 0, "athnfw", SEC_TO_NSEC(1));

Perhaps there is something I am not understanding here? Please advise. 
Thanks!

Farhan