diff --git a/include/Makefile.am b/include/Makefile.am
index 4999ee9..673446f 100644
--- a/include/Makefile.am
+++ b/include/Makefile.am
@@ -1,5 +1,6 @@
 noinst_HEADERS =
 noinst_HEADERS += string.h
+noinst_HEADERS += sys/_null.h
 noinst_HEADERS += sys/queue.h
 noinst_HEADERS += sys/tree.h
 noinst_HEADERS += sys/types.h