xmlDocument.loadXml/load raises NotImplementedException on android, works in editor

I want to create an xml file and read from it when the application starts, and populate it with new info during runtime. This can't be an asset because I want to write to it.

I'm using PropertyListSerialzer given here.

It works just fine without changes in the Editor, but when I deploy to a device, I get a not yet implemented exception when the library code calls XmlDocument.LoadXml in LoadPlistFromFile.

Here is how I'm calling the method:

 var PLIST_STRING : String = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n +
             <!DOCTYPE plist PUBLIC \"-//Apple//DTD PLIST 1.0//EN\" +
             \"http://www.apple.com/DTDs/PropertyList-1.0.dtd\">\n<plist + 
             version=\"1.0\">\n<dict>\n  </dict>\n</plist>";
 var plist : Hashtable = new Hashtable();
 var xmlFile = Application.persistentDataPath + "/" + "bnp_defaults.plist";
 if(!File.Exists(xmlFile)){
     var sw : StreamWriter = new StreamWriter(xmlFile,true);
     sw.Write(CHoneyLizerData.PLIST_STRING);
     sw.Close();
 }
 if(PropertyListSerializer.LoadPlistFromFile(xmlFile, plist)){
     //code...
 }