Sharing my calendar with a non-Zoho user

I'm trying to share my calendar with somebody who is not a Zoho user. This help page says: "This need not be a Zoho email address." However, when I shared my calendar with somebody with a non-Zoho email address, the person replied stating they could not view my calendar because they don't have a Zoho account. Are they doing something wrong? If not, how do I share my calendar with somebody outside Zoho without requiring them to obtain an account? Thanks...