diff --git a/src/platform/windows/win_io.c b/src/platform/windows/win_io.c
index 1e985130b..3351b9ddb 100644
--- a/src/platform/windows/win_io.c
+++ b/src/platform/windows/win_io.c
@@ -23,6 +23,8 @@ static int      win_io_nthr = 0;
 static HANDLE   win_io_h    = NULL;
 static nni_thr *win_io_thrs;
 
+static SRWLock win_io_lock = SRWLOCK_INIT;
+
 static void
 win_io_handler(void *arg)
 {
@@ -45,9 +47,15 @@ win_io_handler(void *arg)
 			break;
 		}
 
+		// The following is done under the win_io_lock to prevent
+		// a use after free race against nni_win_io_fini.
+		AcquireSRWLockShared(&win_io_lock);
 		item = CONTAINING_RECORD(olpd, nni_win_io, olpd);
-		rv   = ok ? 0 : nni_win_error(GetLastError());
-		item->cb(item, rv, (size_t) cnt);
+		if (item->cb != NULL) {
+			rv = ok ? 0 : nni_win_error(GetLastError());
+			item->cb(item, rv, (size_t) cnt);
+		}
+		ReleaseSRWLockShared(&win_io_lock);
 	}
 }
 
@@ -61,26 +69,44 @@ nni_win_io_register(HANDLE h)
 }
 
 int
-nni_win_io_init(nni_win_io *io, nni_win_io_cb cb, void *ptr)
+nni_win_io_init(nni_win_io *io, HANDLE h, nni_win_io_cb cb, void *ptr)
 {
 	ZeroMemory(&io->olpd, sizeof(io->olpd));
 
-	io->cb          = cb;
-	io->ptr         = ptr;
-	io->aio         = NULL;
-	io->olpd.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
-	if (io->olpd.hEvent == NULL) {
-		return (nni_win_error(GetLastError()));
-	}
+	io->f   = h;
+	io->cb  = cb;
+	io->ptr = ptr;
+	io->aio = NULL;
 	return (0);
 }
 
 void
 nni_win_io_fini(nni_win_io *io)
 {
-	if (io->olpd.hEvent != NULL) {
-		CloseHandle((HANDLE) io->olpd.hEvent);
+	DWORD num;
+	// Make absolutely sure there is no I/O running.
+	if (io->f != INVALID_HANDLE) {
+		CancelIoEx(io->f, &o->olpd);
+		(void) GetOverlappedResult(io->f, &io->olpd, &num, TRUE);
 	}
+
+	// Now  acquire the win_io_lock to make sure that no threads are
+	// running in this loop.  Note that there is a subtle race here, where
+	// in theory its possible for the callback thread to be just about to
+	// to start the callback (entering this lock).  This is very narrow,
+	// and it's unlikely to be problematic if it does, because we do not
+	// immedately destroy the data behind the overlapped I/O. Closing this
+	// race requires stopping all of the threads in the thread pool (which
+	// means waking them all up).  This may be worth doing in the future.
+	// One problem with this approach is that we have to have a way to wake
+	// *all* of them, not just one.
+	//
+	// One approach to doing this would be to close the handle of the
+	// completion port itself, and generate a new one.  This seems likely
+	// to be rather expensive?
+	AcquireSRWLockExclusive(&win_io_lock);
+	io->cb = NULL;
+	ReleaseSRWLockExclusive(&win_io_lock);
 }
 
 int
diff --git a/src/platform/windows/win_ipcconn.c b/src/platform/windows/win_ipcconn.c
index abe92604b..86dd842be 100644
--- a/src/platform/windows/win_ipcconn.c
+++ b/src/platform/windows/win_ipcconn.c
@@ -422,8 +422,8 @@ nni_win_ipc_init(
 	c->stream.s_get   = ipc_get;
 	c->stream.s_set   = ipc_set;
 
-	if (((rv = nni_win_io_init(&c->recv_io, ipc_recv_cb, c)) != 0) ||
-	    ((rv = nni_win_io_init(&c->send_io, ipc_send_cb, c)) != 0)) {
+	if (((rv = nni_win_io_init(&c->recv_io, p, ipc_recv_cb, c)) != 0) ||
+	    ((rv = nni_win_io_init(&c->send_io, p, ipc_send_cb, c)) != 0)) {
 		ipc_free(c);
 		return (rv);
 	}
diff --git a/src/platform/windows/win_ipclisten.c b/src/platform/windows/win_ipclisten.c
index 07969e974..a8f60a190 100644
--- a/src/platform/windows/win_ipclisten.c
+++ b/src/platform/windows/win_ipclisten.c
@@ -17,7 +17,7 @@
 
 typedef struct {
 	nng_stream_listener sl;
-	char *              path;
+	char               *path;
 	bool                started;
 	bool                closed;
 	HANDLE              f;
@@ -33,7 +33,7 @@ typedef struct {
 static void
 ipc_accept_done(ipc_listener *l, int rv)
 {
-	nni_aio *   aio;
+	nni_aio    *aio;
 	HANDLE      f;
 	nng_stream *c;
 
@@ -138,7 +138,7 @@ static int
 ipc_listener_set_sec_desc(void *arg, const void *buf, size_t sz, nni_type t)
 {
 	ipc_listener *l = arg;
-	void *        desc;
+	void         *desc;
 	int           rv;
 
 	if ((rv = nni_copyin_ptr(&desc, buf, sz, t)) != 0) {
@@ -200,7 +200,7 @@ ipc_listener_listen(void *arg)
 	ipc_listener *l = arg;
 	int           rv;
 	HANDLE        f;
-	char *        path;
+	char         *path;
 
 	nni_mtx_lock(&l->mtx);
 	if (l->started) {
@@ -331,7 +331,8 @@ nni_ipc_listener_alloc(nng_stream_listener **lp, const nng_url *url)
 	if ((l = NNI_ALLOC_STRUCT(l)) == NULL) {
 		return (NNG_ENOMEM);
 	}
-	if ((rv = nni_win_io_init(&l->io, ipc_accept_cb, l)) != 0) {
+	if ((rv = nni_win_io_init(&l->io, INVALID_HANDLE, ipc_accept_cb, l)) !=
+	    0) {
 		NNI_FREE_STRUCT(l);
 		return (rv);
 	}
diff --git a/src/platform/windows/win_tcpconn.c b/src/platform/windows/win_tcpconn.c
index fad7c495b..19d17c393 100644
--- a/src/platform/windows/win_tcpconn.c
+++ b/src/platform/windows/win_tcpconn.c
@@ -448,8 +448,8 @@ nni_win_tcp_init(nni_tcp_conn **connp, SOCKET s)
 	c->ops.s_get   = tcp_get;
 	c->ops.s_set   = tcp_set;
 
-	if (((rv = nni_win_io_init(&c->recv_io, tcp_recv_cb, c)) != 0) ||
-	    ((rv = nni_win_io_init(&c->send_io, tcp_send_cb, c)) != 0) ||
+	if (((rv = nni_win_io_init(&c->recv_io, s, tcp_recv_cb, c)) != 0) ||
+	    ((rv = nni_win_io_init(&c->send_io, s, tcp_send_cb, c)) != 0) ||
 	    ((rv = nni_win_io_register((HANDLE) s)) != 0)) {
 		tcp_free(c);
 		return (rv);
diff --git a/src/platform/windows/win_tcpdial.c b/src/platform/windows/win_tcpdial.c
index b4a6d7459..479562389 100644
--- a/src/platform/windows/win_tcpdial.c
+++ b/src/platform/windows/win_tcpdial.c
@@ -208,7 +208,7 @@ nni_tcp_dial(nni_tcp_dialer *d, const nni_sockaddr *sa, nni_aio *aio)
 
 	c->peername = ss;
 
-	if ((rv = nni_win_io_init(&c->conn_io, tcp_dial_cb, c)) != 0) {
+	if ((rv = nni_win_io_init(&c->conn_io, s, tcp_dial_cb, c)) != 0) {
 		nni_aio_finish_error(aio, rv);
 		return;
 	}
diff --git a/src/platform/windows/win_tcplisten.c b/src/platform/windows/win_tcplisten.c
index 7cfef8d24..e1aeddad6 100644
--- a/src/platform/windows/win_tcplisten.c
+++ b/src/platform/windows/win_tcplisten.c
@@ -81,13 +81,13 @@ tcp_listener_funcs(nni_tcp_listener *l)
 static void
 tcp_accept_cb(nni_win_io *io, int rv, size_t cnt)
 {
-	nni_tcp_conn *    c = io->ptr;
+	nni_tcp_conn     *c = io->ptr;
 	nni_tcp_listener *l = c->listener;
-	nni_aio *         aio;
+	nni_aio          *aio;
 	int               len1;
 	int               len2;
-	SOCKADDR *        sa1;
-	SOCKADDR *        sa2;
+	SOCKADDR         *sa1;
+	SOCKADDR         *sa2;
 	BOOL              nd;
 	BOOL              ka;
 
@@ -270,7 +270,7 @@ static void
 tcp_accept_cancel(nni_aio *aio, void *arg, int rv)
 {
 	nni_tcp_listener *l = arg;
-	nni_tcp_conn *    c;
+	nni_tcp_conn     *c;
 
 	nni_mtx_lock(&l->mtx);
 	if ((c = nni_aio_get_prov_data(aio)) != NULL) {
@@ -322,7 +322,7 @@ nni_tcp_listener_accept(nni_tcp_listener *l, nni_aio *aio)
 	c->listener = l;
 	c->conn_aio = aio;
 	nni_aio_set_prov_data(aio, c);
-	if (((rv = nni_win_io_init(&c->conn_io, tcp_accept_cb, c)) != 0) ||
+	if (((rv = nni_win_io_init(&c->conn_io, s, tcp_accept_cb, c)) != 0) ||
 	    ((rv = nni_aio_schedule(aio, tcp_accept_cancel, l)) != 0)) {
 		nni_aio_set_prov_data(aio, NULL);
 		nni_mtx_unlock(&l->mtx);
diff --git a/src/platform/windows/win_udp.c b/src/platform/windows/win_udp.c
index 88f2cd5b8..55148ceac 100644
--- a/src/platform/windows/win_udp.c
+++ b/src/platform/windows/win_udp.c
@@ -38,7 +38,7 @@ static void udp_recv_start(nni_plat_udp *);
 int
 nni_plat_udp_open(nni_plat_udp **udpp, nni_sockaddr *sa)
 {
-	nni_plat_udp *   u;
+	nni_plat_udp    *u;
 	SOCKADDR_STORAGE ss;
 	int              sslen;
 	DWORD            no;
@@ -67,7 +67,7 @@ nni_plat_udp_open(nni_plat_udp **udpp, nni_sockaddr *sa)
 	(void) setsockopt(
 	    u->s, IPPROTO_IPV6, IPV6_V6ONLY, (char *) &no, sizeof(no));
 
-	if (((rv = nni_win_io_init(&u->rxio, udp_recv_cb, u)) != 0) ||
+	if (((rv = nni_win_io_init(&u->rxio, u->s, udp_recv_cb, u)) != 0) ||
 	    ((rv = nni_win_io_register((HANDLE) u->s)) != 0)) {
 		nni_plat_udp_close(u);
 		return (rv);
@@ -115,10 +115,10 @@ nni_plat_udp_send(nni_plat_udp *u, nni_aio *aio)
 {
 	SOCKADDR_STORAGE to;
 	int              tolen;
-	nng_sockaddr *   sa;
+	nng_sockaddr    *sa;
 	unsigned         naiov;
-	nni_iov *        aiov;
-	WSABUF *         iov;
+	nni_iov         *aiov;
+	WSABUF          *iov;
 	int              rv;
 	DWORD            nsent;
 
@@ -191,7 +191,7 @@ udp_recv_cb(nni_win_io *io, int rv, size_t num)
 {
 	nni_plat_udp *u = io->ptr;
 	nni_sockaddr *sa;
-	nni_aio *     aio;
+	nni_aio      *aio;
 
 	nni_mtx_lock(&u->lk);
 	if ((aio = nni_list_first(&u->rxq)) == NULL) {
@@ -226,7 +226,7 @@ udp_recv_start(nni_plat_udp *u)
 	DWORD    flags;
 	nni_iov *aiov;
 	unsigned naiov;
-	WSABUF * iov;
+	WSABUF  *iov;
 	nni_aio *aio;
 
 	if ((u->s == INVALID_SOCKET) || (u->closed)) {