Xpra: Ticket #2334: cannot concatenate 'str' and 'memoryview' objects

xpra GTK2 X11 version 2.5-r22138 64-bit
 running with pid 1 on Linux CentOS Linux 7.6.1810 Core
HTML5 Posix Firefox client version 3.0

(Note that I'm using HTML5 client version 3.0 with Xpra version 2.5)

2019-06-15 23:10:43,923 Error: read on ws socket: 127.0.0.1:8080 <- 127.0.0.1:33300 failed: <type 'exceptions.TypeError'>
Traceback (most recent call last):
  File "/usr/lib64/python2.7/site-packages/xpra/net/protocol.py", line 638, in _io_thread_loop
    while not self._closed and callback():
  File "/usr/lib64/python2.7/site-packages/xpra/net/protocol.py", line 717, in _read
    self._process_read(buf)
  File "/usr/lib64/python2.7/site-packages/xpra/net/websockets/protocol.py", line 160, in parse_ws_frame
    self._process_ws_ping(full_payload)
  File "/usr/lib64/python2.7/site-packages/xpra/net/websockets/protocol.py", line 169, in _process_ws_ping
    item = encode_hybi_header(OPCODE_PONG, len(payload)) + payload
TypeError: cannot concatenate 'str' and 'memoryview' objects
2019-06-15 23:10:43,932 xpra client 1 disconnected.