diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index 6d2634c257..739f7cd225 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -282,7 +282,10 @@ def normalize(img):
 def quantize(img, num_of_colors=256, dither=True):
     if imageops is None:
         raise RuntimeError(imageops_err)
-    return imageops.quantize(image_from_data(img), num_of_colors, dither)
+    img = image_from_data(img)
+    if img.hasAlphaChannel():
+        img = blend_image(img)
+    return imageops.quantize(img, num_of_colors, dither)
 
 def run_optimizer(file_path, cmd, as_filter=False, input_data=None):
     file_path = os.path.abspath(file_path)