Error The specified pixel format is not supported. using GifWriter - GifWriter throws error

I get the error "The specified pixel format is not supported." on the line: Pipeline.Run(reader + writer);
using (var writer = new GifWriter(gifFlyerPath))
{
writer.FrameOptions.Delay = 200;
for (int i = 0; i < pngFiles.Count; i++)
{
using (var reader = ImageReader.Create(pngFiles[i]))
{
Pipeline.Run(reader + writer);
}
}
}
The png files are created in WPF by rendering a Visual using:
RenderTargetBitmap bmr = new RenderTargetBitmap(
(int) adjustedWidth
, (int) adjustedHeight
, resolut
, resolut
, PixelFormats.Pbgra32);

bmr.Render(ThisDocument.Canvas_Base);

Bitmap bm = null;
using (MemoryStream stream = new MemoryStream())
{
BitmapEncoder encoder = new PngBitmapEncoder();
encoder.Frames.Add(BitmapFrame.Create(bmr));
encoder.Save(stream);
bm = new Bitmap(stream);
var parms = new EncoderParameters(1);
parms.Param[0] = new EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);

string thisSlidePath = slidePath + sNo + ".png";
bm.Save(thisSlidePath , GetEncoder(ImageFormat.Png), parms);
pngFiles.Add(thisSlidePath);
sNo++;
}

The sample you referenced gave me a solution. Thank you.