goblin not working with aiohttp (server) about goblin HOT 2 OPEN

aiogremlin==3.2.6rc1
aiohttp==2.2.5
yarl==0.18.0

/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/aiohttp/http_writer.py:317: UserWarning: strict parameter is ignored
  path = yarl.quote(path, safe='@:', protected='/', strict=False)
Error handling request
Traceback (most recent call last):
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/aiohttp/web_protocol.py", line 422, in start
    resp = yield from self._request_handler(request)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/aiohttp/web.py", line 306, in _handle
    resp = yield from handler(request)
  File "start.py", line 33, in websocket_handler
    await session.flush()
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 242, in flush
    await self.save(elem)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 292, in save
    result = await self.save_vertex(elem)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 311, in save_vertex
    self._update_vertex)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 412, in _save_element
    result = await create_func(elem)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 419, in _add_vertex
    return await self._add_properties(traversal, props, vertex)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 473, in _add_properties
    return await self._simple_traversal(traversal, elem)
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/goblin/session.py", line 388, in _simple_traversal
    elem = await traversal.next()
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/aiogremlin/process/graph_traversal.py", line 56, in next
    return await self.__anext__()
  File "/home/user/.local/share/virtualenvs/aiohttp-server-janusgraph-xqO7jeAA/lib/python3.6/site-packages/aiogremlin/process/graph_traversal.py", line 18, in __anext__
    object = self.last_traverser.object
AttributeError: 'str' object has no attribute 'object'