Can't run mutmut

I have this error.

command: mutmut run --paths-to-mutate=$PROJECT --tests-dir=$TESTS_DIR

Traceback (most recent call last):
  File "/usr/local/bin/mutmut", line 5, in <module>
    climain()
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 695, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.5/site-packages/click/core.py", line 654, in main
    _verify_python3_env()
  File "/usr/local/lib/python3.5/site-packages/click/_unicodefun.py", line 69, in _verify_python3_env
    if locale.lower().endswith(('.utf-8', '.utf8')):
TypeError: a bytes-like object is required, not 'str'

Docker:

Client:
 Version:      18.03.1-ce
 API version:  1.37
 Go version:   go1.9.5
 Git commit:   9ee9f40
 Built:        Thu Apr 26 07:17:20 2018
 OS/Arch:      linux/amd64
 Experimental: false
 Orchestrator: swarm

Server:
 Engine:
  Version:      18.03.1-ce
  API version:  1.37 (minimum version 1.12)
  Go version:   go1.9.5
  Git commit:   9ee9f40
  Built:        Thu Apr 26 07:15:30 2018
  OS/Arch:      linux/amd64
  Experimental: false

Docker Image: amazonlinux:2017.09

requirements.txt:

...
mutmut==1.3.1
pytest==4.2.0

[JamieYnonan]

Thanks @boxed , I changed the command: mutmut run --paths-to-mutate=$PROJECT --tests-dir=$TESTS_DIR --runner="pytest"

I also had a class that gave me problems.

Thanks.

[boxed]

I’d recommend to put those settings in setup.cfg so you don’t need to remember them and can just run mutmut run.