Share IP don't work with subdomains

[udoL]

like server-a.sub.domain.org share an IP with server-b.sub.domain.org.

Select Share IP -> find "server-b.sub" and get the error message:
Save failed. ERROR => The host specified, server-b.sub, does not exist!
If I switch the server to server-a.domain.org the "Share IP"-Funktion work.