diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 0118abd..5a13f4e 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -86,7 +86,7 @@
 #define DFL_MAX_PATHLEN         -1
 #define DFL_SIG_ALG             MBEDTLS_MD_SHA256
 #define DFL_KEY_USAGE           0
-#define DFL_EXT_KEY_USAGE       0
+#define DFL_EXT_KEY_USAGE       NULL
 #define DFL_NS_CERT_TYPE        0
 #define DFL_VERSION             3
 #define DFL_AUTH_IDENT          1