TypeError: '<' not supported between instances of 'int' and 'NoneType'

[selasphoruskershaw]

Hi,

I am trying to run Tajima's D in vcf-kit using the following command:
docker run -it andersenlab/vcf-kit vk tajima 10000 --sliding file.vcf

However, I receive the following set of error messages immediately:
Traceback (most recent call last):
File "/opt/conda/envs/vcf-kit/lib/python3.7/site-packages/vcfkit/tajima.py", line 148, in
main()
File "/opt/conda/envs/vcf-kit/lib/python3.7/site-packages/vcfkit/tajima.py", line 129, in main
if wz < sz:
TypeError: '<' not supported between instances of 'int' and 'NoneType'

I have tried different vcf files to see if that might have been the problem, but I get the same error message regardless. I am running Python version 3.6.13 in Terminal in Mac BigSur. What can I do to fix this? Thanks for your help!