diff --git a/lib/libcasper/libcasper/libcasper_impl.c b/lib/libcasper/libcasper/libcasper_impl.c
index f58bd44d2039..8d1ebd8dd074 100644
--- a/lib/libcasper/libcasper/libcasper_impl.c
+++ b/lib/libcasper/libcasper/libcasper_impl.c
@@ -1,71 +1,72 @@
 /*-
  * Copyright (c) 2013 The FreeBSD Foundation
  * Copyright (c) 2015 Mariusz Zaborski <oshogbo@FreeBSD.org>
  * All rights reserved.
  *
  * This software was developed by Pawel Jakub Dawidek under sponsorship from
  * the FreeBSD Foundation.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHORS AND CONTRIBUTORS ``AS IS'' AND
  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHORS OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  *
  * $FreeBSD$
  */
 
 #include <err.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <paths.h>
 #include <stdlib.h>
+#include <unistd.h>
 
 #include "libcasper_impl.h"
 
 bool
 fd_is_valid(int fd)
 {
 
 	return (fcntl(fd, F_GETFL) != -1 || errno != EBADF);
 }
 
 void
 fd_fix_environment(int *fdp)
 {
 	int nullfd, nfd;
 
 	if (*fdp > STDERR_FILENO)
 		return;
 
 	nullfd = open(_PATH_DEVNULL, O_RDWR);
 	if (nullfd == -1)
 		errx(1, "Unable to open %s", _PATH_DEVNULL);
 
 	while (*fdp <= STDERR_FILENO) {
 		nfd = dup(*fdp);
 		if (nfd == -1)
 			errx(1, "Unable to secure fd");
 		if (dup2(nullfd, *fdp) == -1)
 			errx(1, "Unable to secure fd");
 		*fdp = nfd;
 	}
 
 	close(nullfd);
 }