在自定义ConfigurationSection中出现无法识别的元素异常

问题是，正如标题所说，每当我试图从我的.config文件加载自定义CustomConfiguration节时，我都会得到一个异常。我尝试加载的app.config文件如下所示:

....
<configSections>
    <sectionGroup name="MainSection">
        ...
        <section name="Directories" type="MyNamespace.DirectoriesSettings, Assembly"/>
    </sectionGroup>
</configSections>
....
<MainSection>
    ...
    <Directories Count="1">
      <Directory id="1" Path="Some'Path" Working="And'Another'Path" Country="US"/>
    </Directories>
</MainSection>
....

和代码

namespace MyNamespace
{
    internal class DirectoriesSettings : ConfigurationSection
    {
        [ConfigurationProperty("Count")]
        public int Count { get { return Convert.ToInt32(base["Count"]); } }
        [ConfigurationProperty("Directories", IsDefaultCollection = true, IsRequired = true)]
        [ConfigurationCollection(typeof(DirectoryElement), AddItemName = "Directory", CollectionType = ConfigurationElementCollectionType.BasicMap)]
        public DirectoryElementCollection Directories { get { return (DirectoryElementCollection)base["Directories"]; } }
    }
    internal class DirectoryElementCollection : ConfigurationElementCollection
    {        
        protected override ConfigurationElement CreateNewElement()
        {
            return new DirectoryElement();
        }
        protected override object GetElementKey(ConfigurationElement element)
        {
            return ((DirectoryElement)element).Id;
        }
    }
    internal class DirectoryElement : ConfigurationElement
    {
        [ConfigurationProperty("id", IsKey = true, IsRequired = true)]
        public int Id
        {
            get { return Convert.ToInt32(this["id"]); }
            set { this["id"] = value; }
        }
        [ConfigurationProperty("Path", IsKey = false, IsRequired = true)]
        public string Path
        {
            get { return Convert.ToString(this["Path"]); }
            set { this["Path"] = value; }
        }
        [ConfigurationProperty("Working", IsKey = false, IsRequired = true)]
        public string Working
        {
            get { return Convert.ToString(this["Working"]); }
            set { this["Working"] = value; }
        }
        [ConfigurationProperty("Country", IsKey = false, IsRequired = true)]
        public string Country
        {
            get { return Convert.ToString(this["Country"]); }
            set { this["Country"] = value; }
        }
    }
}

和使用它的代码

class SettingsContainer
{
    private const string PARENT_SECTION = "MainSection";
    ...
    private DirectoriesSettings _directoriesSettings;
    public DirectoriesSettings Directories { get { return _directoriesSettings; } }
    ...

    public SettingsContainer()
    {
        ...
        _directoriesSettings = (DirectoriesSettings)ConfigurationManager.GetSection(string.Format("{0}/Directories", PARENT_SECTION));
        ...
    }
}

当我试图设置_directoressettings变量时，抛出异常(无法识别的元素'Directory')。

我认为我做得对，但显然我遗漏了一些东西。