diff --git a/libraries/liblutil/entropy.c b/libraries/liblutil/entropy.c
index 13e9d9dd25..eb3209ec39 100644
--- a/libraries/liblutil/entropy.c
+++ b/libraries/liblutil/entropy.c
@@ -8,6 +8,7 @@
 
 #include <ac/string.h>
 #include <ac/time.h>
+#include <ac/unistd.h>
 
 #ifdef HAVE_WINCRYPT_H
 #include <wincrypt.h>