diff --git a/openbsc/include/Makefile.am b/openbsc/include/Makefile.am
index 4596b6e..3234e62 100644
--- a/openbsc/include/Makefile.am
+++ b/openbsc/include/Makefile.am
@@ -1,3 +1,8 @@
-SUBDIRS = openbsc
+SUBDIRS = \
+	openbsc \
+	$(NULL)
 
-noinst_HEADERS = mISDNif.h compat_af_isdn.h
+noinst_HEADERS = \
+	mISDNif.h \
+	compat_af_isdn.h \
+	$(NULL)