diff --git a/src/calibre/utils/imageops/ordered_dither.cpp b/src/calibre/utils/imageops/ordered_dither.cpp
index abf9c46c60..dcf1be5d82 100644
--- a/src/calibre/utils/imageops/ordered_dither.cpp
+++ b/src/calibre/utils/imageops/ordered_dither.cpp
@@ -70,9 +70,9 @@ static uint8_t
 QImage ordered_dither(const QImage &image) { // {{{
     ScopedGILRelease PyGILRelease;
     QImage img = image;
-    QRgb *row = NULL, *pixel = NULL;
     int y = 0, x = 0, width = img.width(), height = img.height();
     uint8_t gray = 0, dithered = 0;
+    QImage dst(width, height, QImage::Format_Indexed8);
 
     // We're running behind blend_image, so, we should only ever be fed RGB32 as input...
     if (img.format() != QImage::Format_RGB32) {
@@ -81,13 +81,15 @@ QImage ordered_dither(const QImage &image) { // {{{
     }
 
     for (y = 0; y < height; y++) {
-        row = reinterpret_cast<QRgb*>(img.scanLine(y));
-        for (x = 0, pixel = row; x < width; x++, pixel++) {
+        const QRgb *src_row = reinterpret_cast<const QRgb*>(img.constScanLine(y));
+        uint8_t *dst_row = dst.scanLine(r);
+        for (x = 0; x < width; x++) {
+            const QRgb pixel = *(src_row + x);
             // We're running behind grayscale_image, so R = G = B
             gray = qRed(*pixel);
             dithered = dither_o8x8(x, y, gray);
-            *pixel = qRgb(dithered, dithered, dithered);
+            *(dst_row + x) = dithered;
         }
     }
-    return img;
+    return dst;
 } // }}}
diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index cf4b26a9d7..d66cf20753 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -463,10 +463,14 @@ def quantize_image(img, max_colors=256, dither=True, palette=''):
 def eink_dither_image(img):
     ''' Dither the source image down to the eInk palette of 16 shades of grey,
     using ImageMagick's OrderedDither algorithm.
+
+    NOTE: Expects input as a grayscale image in RGB32 pixel format (as returned by grayscale_image).
+          Running blend_image if the image has an alpha channel,
+          or grayscale_image if it's not already grayscaled is the caller's responsibility.
+
+    Returns a QImage in Indexed8 pixel format.
     '''
     img = image_from_data(img)
-    if img.hasAlphaChannel():
-        img = blend_image(img)
     return imageops.ordered_dither(img)
 
 # }}}