how to build inverted index

[liulfy]

When I try to build inverted index using
ensureIndex(index_type='inverted', fields=[ "_key" ])
It will raise exception
Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/home/lib/python3.8/site-packages/pyArango/collection.py", line 624, in ensureIndex self.indexes[index_type][ind.infos["id"]] = ind KeyError: 'inverted'

It didn't fail in collection.py line 623, which means there is an index named 'inverted'.
How to create this index?