diff --git a/openbsc/doc/Makefile.am b/openbsc/doc/Makefile.am
index aee2d7b..5a23107 100644
--- a/openbsc/doc/Makefile.am
+++ b/openbsc/doc/Makefile.am
@@ -1 +1,3 @@
-SUBDIRS = examples
+SUBDIRS = \
+	examples \
+	$(NULL)