parseExpression() loses ? information

[vegemite4me]

Thank you for producing this very useful library. I think this is a bug, but I am happy to be corrected.

I have a cron expression with a question mark in it:
0 0 17 * * ?

No problems parsing it, but if I stringify it without making any changes, I expect to get the same cron. But instead the ? is converted to a * and it becomes:
0 0 17 * * *

const result = parseExpression("0 0 17 * * ?");
console.log(result.stringify(true));  // prints "0 0 17 * * *"

How can I parse such an expression, and stringify it later to get back the original specification?