Got TypeError: unhashable type: 'list' when running eval_mrr.py

[ccheng16]

I got the error

Traceback (most recent call last):
File "src/scripts/retrieval/document/eval_mrr.py", line 22, in
evidence = set([t[1] for t in js["evidence"] if isinstance(t,list) and len(t)>1])
TypeError: unhashable type: 'list'

when running

python src/scripts/retrieval/document/eval_mrr.py --split dev --count 5

It seems that the t[1] can be a list, but isn't it supposed to be a string type?