--- sslproto.py.orig	2020-08-04 19:04:51.323355949 +0900
+++ sslproto.py	2020-08-18 14:56:32.573417135 +0900
@@ -11,6 +11,8 @@
 from . import transports
 from .log import logger
 
+# bugfix: IndexError deque index out of range
+import threading
 
 def _create_transport_context(server_side, server_hostname):
     if server_side:
@@ -454,6 +456,8 @@
         self._transport = None
         self._call_connection_made = call_connection_made
         self._ssl_handshake_timeout = ssl_handshake_timeout
+        # bugfix: IndexError deque index out of range
+        self._write_sync = threading.Lock()
 
     def _set_app_protocol(self, app_protocol):
         self._app_protocol = app_protocol
@@ -587,9 +591,11 @@
             self._write_appdata(b'')
 
     def _write_appdata(self, data):
-        self._write_backlog.append((data, 0))
-        self._write_buffer_size += len(data)
-        self._process_write_backlog()
+        # bugfix: IndexError deque index out of range
+        with self._write_sync:
+            self._write_backlog.append((data, 0))
+            self._write_buffer_size += len(data)
+            self._process_write_backlog()
 
     def _start_handshake(self):
         if self._loop.get_debug():