Why return a default constructed ip address on failure?

[mclow]

[internet.address.v4.creation]/6 says (about make_address_v4):

Returns: If successful, an address_v4 value corresponding to the string str. Otherwise address_v4().

What is the benefit for returning all zeros?
Why not just say something like "Otherwise, the return value is unspecified".

When I'm parsing these bits, if I get a failure, I have to either (1) zero the internal array, or (2) return a different value, which defeats some cases of RVO.
(Obviously, this is not specific to address_v4]