diff --git a/ares/setup.h b/ares/setup.h
index b7b99b5e3..3a10745c2 100644
--- a/ares/setup.h
+++ b/ares/setup.h
@@ -49,7 +49,7 @@ typedef int ares_socket_t;
 #endif
 
 #if !defined(HAVE_UNISTD_H) && !defined(_MSC_VER)
-#define HAVE_UNISTD_H
+#define HAVE_UNISTD_H 1
 #endif
 
 #if !defined(HAVE_SYS_UIO_H) && !defined(WIN32) && !defined(MSDOS)