"No JSON object" error when calling new_har() with parameters

[flesheatingarthropods]

I am using MacOS 10.12.6. with the preinstalled Python 2.7.10. and browsermobproxy 2.1.4.

browsermobproxy-py is version 0.8.0 installed via pip.

I trying to create har files with the following code (which mostly works):

from browsermobproxy import Server
server = Server("browsermob-proxy/bin/browsermob-proxy")
server.start()
proxy = server.create_proxy()

from selenium import webdriver

chrome_options = webdriver.ChromeOptions()
chrome_options.add_argument("--allow-file-access-from-files");
chrome_options.add_argument("disable-infobars");
chrome_options.add_argument("--proxy-server={0}".format(proxy.proxy))
driver = webdriver.Chrome(chrome_options = chrome_options)

proxy.new_har('someString')
driver.get("https://www.google.de/")
result = proxy.har # returns a HAR JSON blob

server.stop()
driver.quit()

If I understand the documentation correctly then the first argument to new_har() is an arbitrary string to disambiguate multiple har files if I want to record har files from multiple urls in one go. However whenever I supply that argument I get the following output:

Traceback (most recent call last):
File "test.py", line 23, in
result = proxy.har # returns a HAR JSON blob
File "build/bdist.macosx-10.12-intel/egg/browsermobproxy/client.py", line 104, in har
File "/Library/Python/2.7/site-packages/requests-2.18.4-py2.7.egg/requests/models.py", line 892, in json
return complexjson.loads(self.text, **kwargs)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/init.py", line 338, in loads
return _default_decoder.decode(s)
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 366, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/json/decoder.py", line 384, in raw_decode
raise ValueError("No JSON object could be decoded")
ValueError: No JSON object could be decoded

The return value from new_har contains a http status code, and I can see that when I provide the argument I get a 500, which i guess is why I don't have any JSON to be decoded.

If I do not supply any arguments to new_har() I get a valid har file.

From looking at the function definition for new_har() I do not understand why this is happening.

I also see a ton of warnings and error messages in server.log ("illegal reflective access operation","Failed to start Jetty server"), but that happens in both cases, so I am not sure if this is related to my problem.

At the moment I am not sure if I there is a problem with my code or my setup, or if there is a bug in browsermobproxy-py.

I would appreciate any guidance on this.