The way DNS works, there needs to be a reverse PTR record in the DNS database, mapping that IP address to a single hostname, in the first place, or you're out of luck (often many hostnames are mapped, via A and CNAME records, all to the same address; so an explicit PTR record is necessary, to single out just one of them, for your reverse mapping from the address to a canonical name).

Reasonable sites should have reverse PTR records in their DNS database, for all their IP addresses, but not all sites are reasonable in this way.