diff --git a/ssl/tls13_both.c b/ssl/tls13_both.c
index 1c4168c..9e9da44 100644
--- a/ssl/tls13_both.c
+++ b/ssl/tls13_both.c
@@ -232,7 +232,7 @@
         {TLSEXT_TYPE_certificate_timestamp, &have_sct, &sct},
     };
 
-    uint8_t alert;
+    uint8_t alert = SSL_AD_DECODE_ERROR;
     if (!ssl_parse_extensions(&extensions, &alert, ext_types,
                               OPENSSL_ARRAY_SIZE(ext_types),
                               0 /* reject unknown */)) {