爲什麼非空XElements會導致NRE？

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爲什麼我的擴展方法GetVendorForXMLElement（）在我將它傳遞給XElements數據時在NRE中爆發？爲什麼非空XElements會導致NRE？

相同的代碼（除了使用的自定義類，這裏是「供應商」）與其他類/數據一起工作，但不在這裏。我在GetVendorForXMLElement（）方法中得到一個NRE。

private void buttonVendors_Click(object sender, EventArgs e) 
{ 
    IEnumerable<Vendor> vendors = GetCollectionOfVendors(); 
} 

private IEnumerable<Vendor> GetCollectionOfVendors() 
{ 
    ArrayList arrList = FetchDataFromServer("http://localhost:21608/api/vendor/getall/dbill/ppus/42"); 
    String contents = "<Vendors>"; 
    foreach (String s in arrList) 
    { 
     contents += s; 
    } 
    contents += "</Vendors>"; 
    String unwantedPreamble = "<ArrayOfVendor xmlns:i=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns=\"http://schemas.datacontract.org/2004/07/CStore.DomainModels.HHS\">"; 
    contents = contents.Replace(unwantedPreamble, String.Empty); 
    contents = contents.Replace("</ArrayOfVendor>", String.Empty); 
    MessageBox.Show(contents); 
    XDocument xmlDoc = XDocument.Parse(contents); 
    // The result (NRE) is the same with any one of these three "styles" of calling Select() 
    //IEnumerable<Vendor> vendors = xmlDoc.Descendants("Vendor").Select(GetVendorForXMLElement).ToList(); 
    //IEnumerable<Vendor> vendors = xmlDoc.Descendants("Vendor").Select(x => GetVendorForXMLElement(x)); 
    IEnumerable<Vendor> vendors = xmlDoc.Descendants("Vendor").Select<XElement, Vendor>(GetVendorForXMLElement); 
    return vendors; 
} 

private static Vendor GetVendorForXMLElement(XElement vendor) 
{ 
    return new Vendor 
    { 
     CompanyName = vendor.Element("CompanyName").Value, 
     VendorID = vendor.Element("VendorID").Value     
    }; 
} 

public class Vendor 
{ 
    public String CompanyName { get; set; } 
    public String VendorID { get; set; } 
    public String siteNum { get; set; } 
}

有是數據;這是我看到與MessageBox.Show（）之前XDocument.Parse（）調用撥打：

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