TypeError: argument of type 'NoneType' is not iterable

[dosshra]

Hello, I am running:
xpclr --out ~/adna/${CH}_xpclr --format vcf --input ~/adna/${CH}_zav_adna_pas.flt.vcf.gz --popA ~/adna/pop_wew --popB ~/adna/pop_gl --chr ${CH}
and get

Traceback (most recent call last):
File "/lustre1/home/fahima/hsela4/.conda/envs/xp/bin/xpclr", line 196, in
main()
File "/lustre1/home/fahima/hsela4/.conda/envs/xp/bin/xpclr", line 97, in main
g1, g2, positions, genetic_dist = xpclr.util.load_vcf_format_data(
File "/lustre1/home/fahima/hsela4/.conda/envs/xp/lib/python3.9/site-packages/xpclr/util.py", line 131, in load_vcf_format_data
samples1 = get_sample_ids(s1)
File "/lustre1/home/fahima/hsela4/.conda/envs/xp/lib/python3.9/site-packages/xpclr/util.py", line 97, in get_sample_ids
if "," in sample_input:
TypeError: argument of type 'NoneType' is not iterable

What could be the reason?
Thank you