CSIP23 always fires

[carlwilson]

Given this valid package
valid.zip
there are several reports of this error

{
        "ruleId" : "CSIP23",
        "location" : "mets/dmdSec/mdRef[@xlink:type=’simple’]",
        "message" : "mets/dmdSec/mdRef[@xlink:type=’simple’] in Root METS.xml can't be null ",
        "severity" : "Error"
}

But I believe the attributes are legal.