_get_raw_response_socket fails with AttributeError: 'NoneType' object has no attribute 'raw'

docker-py version: 3.7.0
python version: 3.6.6
docker version:

10:04:15 Client:
10:04:15  Version:           18.09.1
10:04:15  API version:       1.39
10:04:15  Go version:        go1.10.6
10:04:15  Git commit:        4c52b90
10:04:15  Built:             Wed Jan  9 19:35:23 2019
10:04:15  OS/Arch:           linux/amd64
10:04:15  Experimental:      false
10:04:15 
10:04:15 Server: Docker Engine - Community
10:04:15  Engine:
10:04:15   Version:          18.09.1
10:04:15   API version:      1.39 (minimum version 1.12)
10:04:15   Go version:       go1.10.6
10:04:15   Git commit:       4c52b90
10:04:15   Built:            Wed Jan  9 19:02:44 2019
10:04:15   OS/Arch:          linux/amd64
10:04:15   Experimental:     false

Code

        # Create database if not exists
        exec_id = self._cli.exec_create(
            container=self.container_name,
            cmd=(
                "bash -c './cockroach sql --insecure"
                " -e \"CREATE DATABASE IF NOT EXISTS {database_name};\"'".format(
                    database_name=database_name,
                )
            ),
        )
        res = self._cli.exec_start(exec_id=exec_id)
        assert self._cli.exec_inspect(exec_id=exec_id)['ExitCode'] == 0, res

Failure

    tests/upgrade/test_cockroach_upgrade.py:51:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    tests/containers/cockroach.py:125: in import_database_from_backup_file
        res = self._cli.exec_start(exec_id=exec_id)
    /venv/lib/python3.6/site-packages/docker/utils/decorators.py:19: in wrapped
        return f(self, resource_id, *args, **kwargs)
    /venv/lib/python3.6/site-packages/docker/api/exec_api.py:168: in exec_start
        return self._read_from_socket(res, stream, tty=tty, demux=demux)
    /venv/lib/python3.6/site-packages/docker/api/client.py:395: in _read_from_socket
        socket = self._get_raw_response_socket(response)
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    self = <docker.api.client.APIClient object at 0x7f9e77f84588>, response = <Response [101]>
        def _get_raw_response_socket(self, response):
            self._raise_for_status(response)
            if self.base_url == "http+docker://localnpipe":
                sock = response.raw._fp.fp.raw.sock
            elif self.base_url.startswith('http+docker://ssh'):
                sock = response.raw._fp.fp.channel
            elif six.PY3:
    >           sock = response.raw._fp.fp.raw
    E           AttributeError: 'NoneType' object has no attribute 'raw'
    /venv/lib/python3.6/site-packages/docker/api/client.py:312: AttributeError

Start in docker/api/client.py at _get_raw_response_socket and follow its caller in docker/api/exec_api.py at exec_start. Reproduce the reported exec_start failure with the supplied environment details, then verify that this response path no longer raises AttributeError when reading the raw socket.