How to handle PROPFIND request with empty <prop> element?

Hi,

 according to RFC4918 one should treat empty PROPFIND requests like an 
<allprop> request, but how should one handle a PROPFIND request which is 
not empty but have an empty <prop> element like:

<?xml version="1.0" encoding="utf-8"?>
   <propfind xmlns="DAV:">
   <prop></prop>
</propfind>

Should this also be treated as an <allprop> request or should we reply 
with say "412 Precondition Failed" or some other error reply?

Sincerely yours, Henrik Holst