text_to_edtf creating invalid edtf string in certain cases

[Hurlyburly2]

>>> result = text_to_edtf('1921 - circa 1970s')
>>> print(result)
1921/197X~
>>> parse_edtf(result)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/dougsimon/projects/collex-models/.venv/lib/python3.9/site-packages/edtf/parser/grammar.py", line 360, in parse_edtf
    raise EDTFParseException(input_string, err) from None
edtf.parser.edtf_exceptions.EDTFParseException: Error at position 9: Invalid input or format near '1921/197X~'. Please provide a valid EDTF string.