diff --git a/tests/Makefile.am b/tests/Makefile.am
index 83524c8b..76534bfa 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -9,7 +9,7 @@
 if HAVE_CUNIT
 
 # picking up the default warning CFLAGS
-AM_CFLAGS = -I$(top_builddir)/include/coap -I$(top_srcdir)/include/coap $(WARNING_CFLAGS) $(CUNIT_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) $(CUNIT_CFLAGS) $(DTLS_CFLAGS) -std=c99
 
 noinst_PROGRAMS = \
  testdriver