Entity Framework Exception when iterating over collection

I have built a simple file manager, that pulls data from two tables, Folders and Files. When you upload a file it gets written to disk on the server. When I need to delete a folder and all of it's sub-folders, sub-folder files and the parent folders files, I select the folder and then click delete, which executes the controller action below.

UPDATE If I change any subsequent call to .ToList() it works.

    [HttpPost]
    public JsonResult DeleteFolder(int folderID)
    {
        var repo = new FileManagerRepository();

        var folder     = repo.GetFolder(folderID);
        var subfolders = repo.GetSubFolders(folderID).ToList();
        var files      = repo.GetFiles(folderID).ToList();

        // delete sub-folders
        if (subfolders.Count() != 0)
        {
            foreach (var subfolder in subfolders)
            {
                // delete sub-folder files
                var subfiles = repo.GetFiles(subfolder.folder_id).ToList();
                if (subfiles.Count() != 0)
                {
                    foreach (var file in subfiles)
                    {
                        repo.DeleteFile(file.file_id);
                        System.IO.File.Delete(Server.MapPath("/content/upload/" + file.file_name));
                    }
                }
                repo.DeleteFolder(subfolder.folder_id);
            }
        }
        // delete files
        if (files.Count() != 0)
        {
            foreach (var file in files)
            {
                repo.DeleteFile(file.file_id);
                System.IO.File.Delete(Server.MapPath("/content/upload" + file.file_name));
            }
        }
        // delete the folder            
        if (folder != null)
        {
            repo.DeleteFolder(folder.folder_id);                
        }
        repo.Save();
        return new JsonResult();
    }

Now here is the FileManagerRepository

 public class FileManagerRepository
{
    private readonly iau_idahoEntities db = new iau_idahoEntities();

    public IQueryable<folders> GetParentFolders()
    {
        return db.folders.Where(f => f.parant_folder_id == 0);
    }

    public IQueryable<folders> GetSubFolders()
    {
        return db.folders.Where(f => f.parant_folder_id != 0);
    }
    public IQueryable<folders> GetSubFolders(int folder_id)
    {
        return db.folders.Where(f => f.parant_folder_id != 0 && f.parant_folder_id == folder_id);
    } 


    public IQueryable<Files> GetFiles(int id)
    {
        return db.Files.Where(f => f.folder_id == id);
    }

    public Files GetFile(int id)
    {
        return db.Files.SingleOrDefault(f => f.file_id == id);
    }

    public void UpdateDownloadCount(int id)
    {
        var file = GetFile(id);
        file.downloaded += 1;         
    }
    public void AddFolder(int parentFolderID, string folderName)
    {
        var folder = new folders { create_date = DateTime.Now, parant_folder_id = parentFolderID, folder_name = folderName };
        db.folders.AddObject(folder);
    }

    public void DeleteFolder(int folder_id)
    {
        var folder = db.folders.SingleOrDefault(f => f.folder_id == folder_id);
        db.folders.DeleteObject(folder);
    }
    public void Save()
    {
        db.SaveChanges();
    }

    public void AddFile(int folder_id,string filename, string description)
    {
        var file = new Files { 
            folder_id = folder_id, 
            file_name = filename, 
            file_extension = filename.Substring(filename.Length - 3, 3), 
            description = description,
            downloaded = 0, 
            physical_name = filename.Substring(0, filename.Length - 4), 
            upload_date = DateTime.Now 
        };
        db.Files.AddObject(file);            
    }

    internal void DeleteFile(int fileID)
    {
        var fileToDelete = db.Files.SingleOrDefault(file => file.file_id == fileID);
        db.Files.DeleteObject(fileToDelete);
    }

    internal folders GetFolder(int folderID)
    {
        return db.folders.SingleOrDefault(folder => folder.folder_id == folderID);
    }
}

The problem is I keep getting the following EF Exception: There is already an open DataReader associated with this Command which must be closed first

I've tried to add MultipleActiveResultSets = true, to my connection string in the web.config, but I still get the exception. I know why this exception is being thrown, but I don't know how to do what I need to and prevent it from throwing an exception.

PLEASE HELP!!!