404 from qualify packets

If you have a SIP server set up as a peer with qualify turned on, Asterisk sends OPTIONS packets like:

OPTIONS sip:example.com SIP/2.0

I noticed in the SIP debugging that this causes 404 packets to come back from one of my peers - is it legal to make an OPTIONS request without a username part? I.e. isn’t it required to send something like:

OPTIONS sip:foo@example.com SIP/2.0

(in the first example, wouldn’t the peer assume that “example.com” was an unqualified address and qualify it to "example.com@example.com"?)