xml binding for ComboBox does not recognize SelectedItem

I bind a XML document in a WPF window. The xml doc looks like this:

<root>
  <common>
    ... more stuff
    <logSeverityLevel>Debug</logSeverityLevel>
    ...
  </common>
  <collections>
    <logSeverityLevels>
      <logSeverityLevel>Debug</logSeverityLevel>
      <logSeverityLevel>Info</logSeverityLevel>
      <logSeverityLevel>Trace</logSeverityLevel>
      <logSeverityLevel>Error</logSeverityLevel>
      <logSeverityLevel>Fatal</logSeverityLevel>
    </logSeverityLevels>
  </collections>
</root>

And the the bindings:

        <WrapPanel FlowDirection="LeftToRight" DataContext="{Binding XPath=/root/common}">

            <DockPanel>
                <TextBlock Text="Log Severity Level : " />
                <ComboBox SelectedItem="{Binding XPath=logSeverityLevel}" ItemsSource="{Binding XPath=/root/collections/logSeverityLevels/*}" />
            </DockPanel>
        </WrapPanel>

Now when I run the program, I can see that the ComboBox is getting populated with the possible items. But it does not set Debug as the selected value. What to do about this?