diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 823b0ccf15..436870b518 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -3389,7 +3389,7 @@ int mbedtls_ssl_prepare_handshake_record(mbedtls_ssl_context *ssl)
         }
         memmove(payload_end, ssl->in_msg, ssl->in_msglen);
 
-        ssl->badmac_seen_or_in_hsfraglen += ssl->in_msglen;
+        ssl->badmac_seen_or_in_hsfraglen += (unsigned) ssl->in_msglen;
         payload_end += ssl->in_msglen;
 
         if (ssl->badmac_seen_or_in_hsfraglen < ssl->in_hslen) {