SimpleAsyncHTTPClient randomly blocking on "ValueError: IOStream is not idle; cannot convert to SSL"

Hi, I'm using SimpleAsyncHTTPClient to perform a few hundred requests per minute. It has been working fine until I started encountering the error below. This error happens randomly (I didn't manage to replicate it) and once it occurs, the SimpleAsyncHTTPClient fails on all his following requests (Timeout while connecting). A process restart solves the problem until the error occurs again. Any idea where this comes from or how to handle it?

I tried catching it and managed to close the client and create a new instance, but it still can't perform HTTP requests until the program is restarted.

ERROR:tornado.application:Exception in callback <functools.partial object at 0x7f6a3894c3c0>
Traceback (most recent call last):
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/ioloop.py", line 758, in _run_callback
    ret = callback()
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/stack_context.py", line 300, in null_wrapper
    return fn(*args, **kwargs)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/ioloop.py", line 779, in _discard_future_result
    future.result()
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1141, in run
    yielded = self.gen.throw(*exc_info)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/simple_httpclient.py", line 272, in run
    max_buffer_size=self.max_buffer_size)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1133, in run
    value = future.result()
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/concurrent.py", line 261, in result
    raise_exc_info(self._exc_info)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/gen.py", line 1147, in run
    yielded = self.gen.send(value)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/tcpclient.py", line 242, in connect
    server_hostname=host)
  File "/home/ubuntu/env/local/lib/python2.7/site-packages/tornado/iostream.py", line 1368, in start_tls
    raise ValueError("IOStream is not idle; cannot convert to SSL")
ValueError: IOStream is not idle; cannot convert to SSL

I'm using tornado 5.1.1 on Python 2.7.15 and here's a snippet of how I initialize the HTTPClient:

    client = AsyncHTTPClient(force_instance=True)
    client.initialize(max_clients=200)

Start with the traceback entries in tornado/simple_httpclient.py, tornado/tcpclient.py, and tornado/iostream.py, especially start_tls and the AsyncHTTPClient initialization shown in the report. Reproduce the failure under high request concurrency if possible and trace why the IOStream is no longer idle; done means the reported SSL conversion failure no longer leaves subsequent requests timing out, with regression coverage for the failure path.