--- a/plugins/mod_websocket.lua	Tue Dec 31 02:50:25 2019 +0100
+++ b/plugins/mod_websocket.lua	Thu Jan 02 10:49:37 2020 +0100
@@ -236,6 +236,7 @@
 			return;
 		elseif opcode == 0x9 then -- Ping frame
 			frame.opcode = 0xA;
+			frame.MASK = false; -- Clients send masked frames, servers don't, see #1484
 			conn:write(build_frame(frame));
 			return "";
 		elseif opcode == 0xA then -- Pong frame, MAY be sent unsolicited, eg as keepalive