Native parser crashes on rf"\\"

[Zac-HD]

import contextlib, os
from libcst import parse_expression

@contextlib.contextmanager
def native_parser():
    var = os.environ.get("LIBCST_PARSER_TYPE")
    try:
        os.environ["LIBCST_PARSER_TYPE"] = "native"
        yield
    finally:
        os.environ.pop("LIBCST_PARSER_TYPE")
        if var is not None:
            os.environ["LIBCST_PARSER_TYPE"] = var

string_literal = r'rf"\\"'
parse_expression(string_literal)  # fine :-)
with native_parser():
    parse_expression(string_literal)  # fails :-(