Fixed enabling the alpha channel for the creation of the merged image.
diff --git a/coders/psd.c b/coders/psd.c
index d03c4ac..aec0ae3 100644
--- a/coders/psd.c
+++ b/coders/psd.c
@@ -1911,6 +1911,7 @@
 
       SetImageAlphaChannel(image,TransparentAlphaChannel,exception);
       image->background_color.alpha=TransparentAlpha;
+      image->background_color.alpha_trait=BlendPixelTrait;
       merged=MergeImageLayers(image,FlattenLayer,exception);
       ReplaceImageInList(&image,merged);
     }