diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 68d91e8..99768a2 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2853,7 +2853,7 @@
     }
 
     /* Forward to version-specific serialization routine. */
-    remaining_len = used <= buf_len ? buf_len - used : 0;
+    remaining_len = (buf_len >= used) ? buf_len - used : 0;
     switch( session->tls_version )
     {
 #if defined(MBEDTLS_SSL_PROTO_TLS1_2)