diff --git a/Makefile.am b/Makefile.am
index 9315de91..921cfa72 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -45,7 +45,7 @@ EXTRA_DIST = \
   win32/testdriver/testdriver.vcxproj.filters \
   win32/testdriver/testdriver.vcxproj.user
 
-AM_CFLAGS = -I$(top_builddir)/include/coap -I$(top_srcdir)/include/coap $(WARNING_CFLAGS) $(DTLS_CFLAGS) -std=c99
+AM_CFLAGS = -I$(top_builddir)/include/coap$(LIBCOAP_API_VERSION) -I$(top_srcdir)/include/coap$(LIBCOAP_API_VERSION) $(WARNING_CFLAGS) $(DTLS_CFLAGS) -std=c99
 
 SUBDIRS = $(subdirs) . $(MAN_DIR) $(DOC_DIR) tests examples