Android JSON parsing on collision with GSON: LinkedTreeMap cannot be cast to object

I am trying to store a list of objects in SharedPreferences, so I am using Gson to convert a list of objects to JSON and back. However, when I save, then retrieve the list of objects, and the ListView adapter is applied to the new list, I get the following error:

java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to appuccino.simplyscan.Objects.Folder
            at appuccino.simplyscan.Extra.DocumentAdapter.getView(DocumentAdapter.java:117)

      

        
        
        
      

    

where the error points to this line in the list:

Folder folder = folderList.get(position);

      

        
        
        
      

    

When storing a list of objects, I use the following:

//folderList is a List<Folder> folderList = new ArrayList<>();
folderList.add(0, newFolder);
Gson gson = new Gson();
String newFoldersJson = gson.toJson(folderList);
PrefManager.putString(PrefManager.FOLDER_JSON, newFoldersJson);

      

        
        
        
      

    

where the last line just stores the line in SharedPreferences. When you fetch the list from SharedPreferences I use the following:

 public static List<Folder> loadFolders(MainActivity main){
        //JSON containing a list of folder objects
        String foldersJSON = PrefManager.getString(PrefManager.FOLDER_JSON, "");
        if(!foldersJSON.isEmpty()){
            Gson gson = new Gson();
            List<Folder> folderList = gson.fromJson(foldersJSON, List.class);
            return folderList;
        }

        return new ArrayList<>();
    }

      

        
        
        
      

    

If that helps, here's how my Folder class is defined:

public class Folder {
    private String name;
    private List<String> docNameList;
    private transient List<Document> docList;

    public Folder(String n) {
        name = n;
        docList = new ArrayList<>();
        docNameList = new ArrayList<>();
    }

}