IndexError

After running the Python test suite, I get this coverage error:

ok
----------------------------------------------------------------------
Ran 59 tests in 17.053s
OK
Traceback (most recent call last):
  File "C:\PYTHON36\Scripts\coverage-script.py", line 11, in <module>
    load_entry_point('coverage==4.0.3', 'console_scripts', 'coverage')()
  File "c:\python36\lib\site-packages\coverage\cmdline.py", line 741, in main
    status = CoverageScript().command_line(argv)
  File "c:\python36\lib\site-packages\coverage\cmdline.py", line 511, in command_line
    **report_args)
  File "c:\python36\lib\site-packages\coverage\control.py", line 1014, in html_report
    return reporter.report(morfs)
  File "c:\python36\lib\site-packages\coverage\html.py", line 135, in report
    self.report_files(self.html_file, morfs, self.config.html_dir)
  File "c:\python36\lib\site-packages\coverage\report.py", line 75, in report_files
    report_fn(fr, self.coverage._analyze(fr))
  File "c:\python36\lib\site-packages\coverage\control.py", line 891, in _analyze
    return Analysis(self.data, it)
  File "c:\python36\lib\site-packages\coverage\results.py", line 28, in __init__
    self._arc_possibilities = sorted(self.file_reporter.arcs())
  File "c:\python36\lib\site-packages\coverage\python.py", line 165, in arcs
    return self.parser.arcs()
  File "c:\python36\lib\site-packages\coverage\parser.py", line 238, in arcs
    for l1, l2 in self.byte_parser._all_arcs():
  File "c:\python36\lib\site-packages\coverage\parser.py", line 624, in _all_arcs
    arcs.update(bp._arcs())
  File "c:\python36\lib\site-packages\coverage\parser.py", line 561, in _arcs
    chunks = self._split_into_chunks()
  File "c:\python36\lib\site-packages\coverage\parser.py", line 493, in _split_into_chunks
    block_stack.pop()
IndexError: pop from empty list
Command executed with exception: Traceback (most recent call last):
  File "C:\PYTHON36\Scripts\coverage-script.py", line 11, in <module>
    load_entry_point('coverage==4.0.3', 'console_scripts', 'coverage')()
  File "c:\python36\lib\site-packages\coverage\cmdline.py", line 741, in main
    status = CoverageScript().command_line(argv)
  File "c:\python36\lib\site-packages\coverage\cmdline.py", line 511, in command_line
    **report_args)
  File "c:\python36\lib\site-packages\coverage\control.py", line 1014, in html_report
    return reporter.report(morfs)
  File "c:\python36\lib\site-packages\coverage\html.py", line 135, in report
    self.report_files(self.html_file, morfs, self.config.html_dir)
  File "c:\python36\lib\site-packages\coverage\report.py", line 75, in report_files
    report_fn(fr, self.coverage._analyze(fr))
  File "c:\python36\lib\site-packages\coverage\control.py", line 891, in _analyze
    return Analysis(self.data, it)
  File "c:\python36\lib\site-packages\coverage\results.py", line 28, in __init__
    self._arc_possibilities = sorted(self.file_reporter.arcs())
  File "c:\python36\lib\site-packages\coverage\python.py", line 165, in arcs
    return self.parser.arcs()
  File "c:\python36\lib\site-packages\coverage\parser.py", line 238, in arcs
    for l1, l2 in self.byte_parser._all_arcs():
  File "c:\python36\lib\site-packages\coverage\parser.py", line 624, in _all_arcs
    arcs.update(bp._arcs())
  File "c:\python36\lib\site-packages\coverage\parser.py", line 561, in _arcs
    chunks = self._split_into_chunks()
  File "c:\python36\lib\site-packages\coverage\parser.py", line 493, in _split_into_chunks
    block_stack.pop()
IndexError: pop from empty list

I post this here, because python-coveralls requires coverage < 4.1, but version >= 4.1 would probably fix above error.
So why is it incompatible and why not upgrade to the new coverage library?