diff --git a/src/lib/w32/posixio.c b/src/lib/w32/posixio.c
index 58dafd92..800295ab 100644
--- a/src/lib/w32/posixio.c
+++ b/src/lib/w32/posixio.c
@@ -269,7 +269,7 @@ posix_listen(int fd, int backlog)
 #undef setsockopt
 int
 posix_setsockopt(int fd, int level, int optname,
-		      const char *optval, int optlen)
+		      const void *optval, int optlen)
 {
     /*
      * SO_REUSEADDR requests from tcp_listen.c
diff --git a/src/lib/w32/unistd.h b/src/lib/w32/unistd.h
index f0e6b970..c556e433 100644
--- a/src/lib/w32/unistd.h
+++ b/src/lib/w32/unistd.h
@@ -128,7 +128,7 @@ extern int posix_accept(int fd, struct sockaddr *addr, socklen_t *addrlen);
 extern int posix_bind(int fd, const struct sockaddr *name, int namelen);
 extern int posix_listen(int fd, int backlog);
 extern int posix_setsockopt(int fd, int level, int optname,
-		      const char *optval, int optlen);
+		      const void *optval, int optlen);
 extern int posix_shutdown(int fd, int how);
 extern int posix_socket(int domain, int type, int protocol);