Creating a dictionary from xml

Given this XML:

<dataroot>
    <TestTable>
        <Tag>tag1</Tag>
        <FName>t1</FName>
    </TestTable>
    <TestTable>
        <Tag>tag2</Tag>
        <FName>t2</FName>
    </TestTable>
    <TestTable>
        <Tag>tag3</Tag>
        <FName>t3</FName>
    </TestTable>
</dataroot>

How can I load it into a Dictionary so that if I look up "Tag3", it returns "t3"?

I have tried:

var doc = XDocument.Load("ConfigItems.xml");
Dictionary<string, string> ConfigItems;
ConfigItems = doc.Descendants("TestTable").ToDictionary(v => v.Attribute("Tag").Value.ToString  (), v => v.Attribute("FName").Value.ToString());

But I get an "Object reference not set to an instance of an object." error.