diff --git a/tests/Makefile.am b/tests/Makefile.am
index 8806d127..162e082e 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -78,6 +78,7 @@ AM_CFLAGS = $(WARNCFLAGS) \
 	-I${top_srcdir}/lib \
 	-I${top_srcdir}/lib/includes \
 	-I${top_builddir}/lib/includes \
+	-DBUILDING_NGHTTP2 \
 	@CUNIT_CFLAGS@ @DEFS@
 
 TESTS = main