diff --git a/src/calibre/utils/imageops/quantize.cpp b/src/calibre/utils/imageops/quantize.cpp
index 5643fc50ee..6787fc9fee 100644
--- a/src/calibre/utils/imageops/quantize.cpp
+++ b/src/calibre/utils/imageops/quantize.cpp
@@ -419,7 +419,7 @@ QImage quantize(const QImage &image, unsigned int maximum_colors, bool dither, c
     if (fmt != QImage::Format_RGB32 && fmt != QImage::Format_Indexed8 && fmt != 24) { // 24 = QImage::Format_Grayscale8
         img = img.convertToFormat(QImage::Format_RGB32); 
         if (img.isNull()) throw std::bad_alloc();
-    }
+    } else if (fmt == 24) img = img.convertToFormat(QImage::Format_Indexed8);
     // There can be no more than MAX_LEAVES * 8 nodes. Add 1 in case there is an off by 1 error somewhere.
     Pool<Node> node_pool((MAX_LEAVES + 1) * 8);  
     if (palette.size() > 0) {