diff --git a/lib/output.c b/lib/output.c
index 3dbe8bb1..656cfafb 100644
--- a/lib/output.c
+++ b/lib/output.c
@@ -127,7 +127,7 @@ int lws_issue_raw(struct lws *wsi, unsigned char *buf, size_t len)
 		lwsl_warn("** error invalid sock but expected to send\n");
 
 	/* limit sending */
-	n = wsi->protocol->rx_buffer_size;
+	n = wsi->protocol->rx_buffer_size + LWS_PRE + 4;
 	if (!n)
 		n = LWS_MAX_SOCKET_IO_BUF;
 	if (n > len)