Yahoo CalDAV ignoring If-None-Match header

Im working with CalDAV, and I'm having a problem. The
Yahoo CalDAV server appears to be completely ignoring the If-None-Match
header. The "If-None-Match: *" request header is supposed to
ensure that the client will not inadvertently overwrite an existing
resource if the last path segment in the resourceURI turns out to
already be used. This is per RFC 4791 - "Calendaring Extensions to
WebDAV (CalDAV)"

The "If-Match: eTag" header works fine, which confuses me as to why
"If-None-Match" is not supported.

Please advise, I really need a way to ensure I am adding a new event and
not updating an existing event by accident. Thanks.

Charles.