Nillable KeyRef element

Hi,

I am trying to specify a KeyRef that may either refer to a value or be
empty. I have found different results with attributes and simple
content. I have validated this with Xerces, XmlSpy and the GotDotNet XSD
Schema Validator and get the same results with all.

ATTRIBUTES

Please refer to the attached test.xsd. A person has an attribute
"manager-id" which refers to another person's "id" attribute. Use of
this attribute is specified as optional. In test.xml the first person
does not have this attribute. This behaves correctly and the parser does
not give an error.

SIMPLE CONTENT

This same xsd specifies that the root element may contain "person"
elements as well as "dog" elements. A person can have a child element
called "dog-name" which refers to a "dog" element. Nillable has been set
to true so that "dog-name" can be empty (nil).

I specifically don't want to remove the "dog-name" element, but just
create an empty "dog-name", as with the second person in test.xml.
Because if the whole element is removed, then there is no place to put
attributes.

I get the following parser error, which means that it doesn't allow the
nil value for "dog-name" even though dog-name has been set to nillable:

Line 13: Parser error: Key 'person.dog-name' with value 'ID Value:  '
not found for identity constraint of element 'personnel'.

Please let me know if there is another way of making an element either
refer to something or be empty.

If there isn't another way, then maybe the spec needs to change so that
if an element is nillable, then the KeyRef doesn't have to refer to a
valid key. Then it will work similar to how attributes work.

Thanks in advance,

Evert