'Malformed Reference Element' when adding a reference based on an Id attribute with SignedXml class

Unable to sign element by Id attribute when there's a namespace prefix:

void Main()
{
    var doc = new XmlDocument();
    doc.LoadXml("<root xmlns:u=\"myuri\"><test u:Id=\"_0\">Zebra</test></root>");

    SignedXml signedXml = new SignedXml(doc);
    signedXml.SigningKey = new RSACryptoServiceProvider();

    Reference reference = new Reference("#_0");
    signedXml.AddReference(reference);

    signedXml.ComputeSignature();
}

ComputeSignature() will fail here with 'Malformed Reference Element' how should this be done?