nillable keyrefs in XSD

As far as I understand, keyrefs in XSD must always point to a valid
key in the instance document; there is no parallel to a C null pointer
or an SQL NULL foreign key, IOW, one cannot say, "this element *may*,
point to another element, but is not required to".

It seems that I can model this SQL in XSD:
FOO INTEGER NOT NULL REFERENCES BAR.BAZ

but not this:
FOO INTEGER NULL REFERENCES BAR.BAZ

Ari.