Forum OpenACS Q&A: Unexpected behaviour of Xowiki field validation for URLs

I have specified a field like so:

{my_url:optional,url,label=My URL}

I expect this to permit form submission without a URL in the field.

However, this throws a validation failure represented by the following message key:

MESSAGE KEY MISSING: 'xowiki.url-validate_safe_url'

I am not sure what this message key is intended to communicate. Is this a message to say that the URL failed the safe-url validation. If so, why, given that this is an optional field.

If I put "" into the field and submit, I get the same:

MESSAGE KEY MISSING: 'xowiki.url-validate_safe_url'

Any ideas?