I have a problem trying to compare 2 xml files with the XmlDiff library from Microsoft.

I am using an overloaded function which passes 2 xmls as strings :

xmldiff = new XmlDiff(XmlDiffOptions.IgnoreChildOrder | XmlDiffOptions.IgnorePrefixes | XmlDiffOptions.IgnoreNamespaces);
bool identical = xmldiff.Compare(first, last, false);

where first and last look more or less like this :

<?xml version="1.0" encoding="ISO-8859-1"?>
<breakfast_menu>
    <food>
        <name>Belgian Waffles</name>
        <price>$5.95</price>
        <description>two of our famous Belgian Waffles with plenty of real maple syrup</description>
        <calories>650</calories>
    </food>
    <food>
        <name>Strawberry Belgian Waffles</name>
        <price>$7.95</price>
        <description>light Belgian waffles covered with strawberries and whipped cream</description>
        <calories>900</calories>
    </food>
</breakfast_menu>

And I get this error