diff --git a/library/ssl_client.c b/library/ssl_client.c
index 82695f1..6d3789d 100644
--- a/library/ssl_client.c
+++ b/library/ssl_client.c
@@ -518,7 +518,11 @@
 #if defined(MBEDTLS_SSL_PROTO_TLS1_2) && defined(MBEDTLS_SSL_PROTO_DTLS)
     if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
     {
-        unsigned char cookie_len = 0;
+#if !defined(MBEDTLS_SSL_PROTO_TLS1_3)
+        uint8_t cookie_len = 0;
+#else
+        uint16_t cookie_len = 0;
+#endif /* !MBEDTLS_SSL_PROTO_TLS1_3 */
 
         if( handshake->cookie != NULL )
         {
@@ -529,7 +533,7 @@
         }
 
         MBEDTLS_SSL_CHK_BUF_PTR( p, end, cookie_len + 1 );
-        *p++ = cookie_len;
+        *p++ = ( unsigned char )cookie_len;
         if( cookie_len > 0 )
         {
             memcpy( p, handshake->cookie, cookie_len );