可访问性字段类型不一致

我的项目中有这个问题。

我已经声明了一个类作为一个单例使用它作为xml解析器。我想返回的点解析列表，但我不能改变返回类型没有得到一个错误，所以我试图声明一个全局变量从其他类读取它，但我收到这个错误。

namespace XML_Parser
{
    public sealed class XMLParserPOI_Wiki
    {
        private List<POI> places;
        private static readonly XMLParserPOI_Wiki uniqueInstance = new XMLParserPOI_Wiki();
        public List<POI> getPlaces()
        {
            return places;
        }
        //Constructor
        private XMLParserPOI_Wiki(){
            System.Diagnostics.Debug.WriteLine("Constructor singleton created");
        }
        public static XMLParserPOI_Wiki getInstance()
        {
            return uniqueInstance;
        }
        public void parseWikitude(string url)
        {
            places = null;
            WebClient wc = new WebClient();
            wc.DownloadStringCompleted += HttpsCompleted;
            wc.DownloadStringAsync(new Uri(url));

        }
        private void HttpsCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                XDocument xdoc = XDocument.Parse(e.Result, LoadOptions.None);
                XNamespace ns = "http://www.opengis.net/kml/2.2";
                XNamespace ns2 = "http://www.openarml.org/wikitude/1.0";
                var placemarkers = xdoc.Root.Descendants(ns + "Placemark");
                places =
                (....).ToList();

            }
        }
    }
}

完整错误是:

Error   1   Inconsistent accessibility: 
field type 'System.Collections.Generic.List<XML_Parser.POI>' is 
less accessible than field 
'XML_Parser.XMLParserPOI_Wiki.places'   
C:'Users'vindi_000'documents'visual studio 2012'Projects'XML_Parser'XML_Parser'XMLParserPOI_Wiki.cs 
13  27  XML_Parser