diff --git a/tests/oss-fuzz/Makefile.ci.in b/tests/oss-fuzz/Makefile.ci.in
index 5af1c895..97e2f837 100644
--- a/tests/oss-fuzz/Makefile.ci.in
+++ b/tests/oss-fuzz/Makefile.ci.in
@@ -15,7 +15,7 @@ WARNING_CFLAGS?=@WARNING_CFLAGS@
 DTLS_CFLAGS?=@DTLS_CFLAGS@
 DTLS_LIBS?=@DTLS_LIBS@
 CPPFLAGS=-I$(top_builddir)/include -I$(top_srcdir)/include -I$(top_srcdir)
-CFLAGS=$(WARNING_CFLAGS) $(DTLS_CFLAGS) -std=c99
+CFLAGS+=$(WARNING_CFLAGS) $(DTLS_CFLAGS) -std=c99
 CFLAGS+=-Wno-missing-prototypes -Wno-missing-declarations
 
 SOURCES:=$(wildcard *_target.c)