ds:RetrievalMethod/@URI optional or mandatory?

There is a discrepancy in section 4.4.3[1] between the schema fragment
and the DTD. The former states that the URI attribute is optional, while
the latter states that it is mandatory. Which is correct? 

I note that the normative schema[2] matches the schema fragment, but
wanted to check. I also checked the errata page, but couldn't see
anything that pertained to this.

Thanks

Martin Gudgin

[1]
http://www.w3.org/TR/2002/REC-xmldsig-core-20020212/#sec-RetrievalMethod
[2] http://www.w3.org/TR/xmldsig-core/xmldsig-core-schema.xsd