diff --git a/library/ssl_tls13_client.c b/library/ssl_tls13_client.c
index f1a21daf2f..80ea06433b 100644
--- a/library/ssl_tls13_client.c
+++ b/library/ssl_tls13_client.c
@@ -309,7 +309,7 @@ static int ssl_tls13_write_key_share_ext( mbedtls_ssl_context *ssl,
         /* Pointer to group */
         unsigned char *group = p;
         /* Length of key_exchange */
-        size_t key_exchange_len;
+        size_t key_exchange_len = 0;
 
         /* Check there is space for header of KeyShareEntry
          * - group                  (2 bytes)