diff --git a/library/ssl_tls13_keys.c b/library/ssl_tls13_keys.c
index dcbfcf0..1967883 100644
--- a/library/ssl_tls13_keys.c
+++ b/library/ssl_tls13_keys.c
@@ -83,7 +83,7 @@
  */
 
 /* We need to tell the compiler that we meant to leave out the null character. */
-static const char tls13_label_prefix[6] __attribute__ ((nonstring)) = "tls13 ";
+static const char tls13_label_prefix[6] MBEDTLS_ATTRIBUTE_UNTERMINATED_STRING = "tls13 ";
 
 #define SSL_TLS1_3_KEY_SCHEDULE_HKDF_LABEL_LEN(label_len, context_len) \
     (2                     /* expansion length           */ \