SystemError: error return without exception set, when using requests and debugger

I had a similar problem when using Gensim Word2vec models, also using debugger in Python 3.6 / PyCharm 2018.2.
Just as a quick fix, I found a solution by setting an environment variable:

PYDEVD_USE_FRAME_EVAL=NO

This can be done easily in PyCharm by settings environment variables in PyCharm run configuration. After setting this variable, I could use debugger again. More info can be found here and here.