Thanks. I am using Ubuntu16.04 with anconda3.5 and i create an virtual environment where python version is 3.6. I fixed it by conda install tornado==5.1.1 in my virtual env. I also use pip install tornado==5.1.1 for more scurity~haha

[imsjz]

Downgrading tornado to 4.5.3 fixed my problem.
with pip - sudo python3.6 -m pip install tornado==4.5.3
with conda - conda install tornado==4.5.3

I m using python 3.6 in virtualenv. fixed it by $ pip install tornado==5.1.1

Great find. Thanks!

Originally posted by @wyattbiker in #2664 (comment)