https://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=31879

diff --git a/MagickCore/channel.c b/MagickCore/channel.c
index 7bf76ba1d04bf4be445e296ab73f695760050976..4792927ed7203f1bfaf12edc2581c75669199af0 100644 (file)
--- a/MagickCore/channel.c
+++ b/MagickCore/channel.c
@@ -1259,6 +1259,9 @@ MagickExport MagickBooleanType SetImageAlphaChannel(Image *image,
       /*
         Set alpha channel by shape.
       */
+      status=SetImageStorageClass(image,DirectClass,exception);
+      if (status == MagickFalse)
+        break;
       image->alpha_trait=UpdatePixelTrait;
       (void) SetImageMask(image,WritePixelMask,image,exception);
       (void) LevelImageColors(image,&image->background_color,