Controls - Why Can't I Store XML In ListBox Value

Why does this work:

ListItem item = new ListItem();
string value = lstAvailExtPropsToFilter.SelectedItem.Text +" = "+ txtExtPropToFilter.Text;
string text = lstAvailExtPropsToFilter.SelectedItem.Text + " = " + txtExtPropToFilter.Text;
item.Text = text;
item.Value = value;
lstExtPropsToFilter.Items.Add(item);

But not this:

ListItem item = new ListItem();
string value = string.Format("<key>{0}</key><value>{1}</value>", lstAvailExtPropsToFilter.SelectedItem.Text, txtExtPropToFilter.Text);
string text = lstAvailExtPropsToFilter.SelectedItem.Text + " = " + txtExtPropToFilter.Text;
item.Text = text;
item.Value = value;
lstExtPropsToFilter.Items.Add(item);