Cannot Access Closed Stream

I'm trying to use the Caching Application Block to cache some images (these images take a long time to render)

  BitmapSource bitmapSource; ///some bitmap source already created
  _cache ///  Caching Application Block
  String someId; //id for this image, used as the key for the cache

  using (var stream = new MemoryStream())
    {
        PngBitmapEncoder encoder = new PngBitmapEncoder();
        encoder.Interlace = PngInterlaceOption.On;
        encoder.Frames.Add(BitmapFrame.Create(bitmapSource));             
        encoder.Save(stream);
        _cache.Add(someId, stream);
    }

And then load them using:

imStream = (Stream)_cache.GetData(someId));
if (imStream != null)
{
    PngBitmapDecoder decoder = new PngBitmapDecoder(imStream,  BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.Default);
    return decoder.Frames[0];  //return the bitmap source
}

But during the load, i get the following exception at that "new PngBitmapDecoder" line:

"Cannot access a closed Stream.

I understand I closed the stream in the above code, but isn't _cache.Add() making a copy (via Serialization) before it exits? What's the correct process of serializing the stream?

THanks!