diff --git a/src/calibre/utils/imageops/imageops.sip b/src/calibre/utils/imageops/imageops.sip
index d76c6b2a99..4bfc05a11a 100644
--- a/src/calibre/utils/imageops/imageops.sip
+++ b/src/calibre/utils/imageops/imageops.sip
@@ -73,10 +73,12 @@ QImage oil_paint(const QImage &image, const float radius=-1, const bool high_qua
 			sipRes = new QImage(oil_paint(*a0, a1, a2));
         IMAGEOPS_SUFFIX
 %End
-QImage quantize(const QImage &image, unsigned int maximum_colors, bool dither, const QVector<QRgb> &palette);
+QImage quantize(const QImage &image, unsigned int maximum_colors, bool dither, SIP_PYTUPLE);
 %MethodCode
+		QVector<QRgb> palette(PyTuple_GET_SIZE(a3));
+		for (int i = 0; i < PyTuple_GET_SIZE(a3); i++) palette[i] = PyLong_AsUnsignedLong(PyTuple_GET_ITEM(a3, i));
         IMAGEOPS_PREFIX
-			sipRes = new QImage(quantize(*a0, a1, a2, *a3));
+			sipRes = new QImage(quantize(*a0, a1, a2, palette));
         IMAGEOPS_SUFFIX
 %End
 
diff --git a/src/calibre/utils/img.py b/src/calibre/utils/img.py
index 85049d99bb..e4b5d54123 100644
--- a/src/calibre/utils/img.py
+++ b/src/calibre/utils/img.py
@@ -503,7 +503,7 @@ def quantize_image(img, max_colors=256, dither=True, palette=''):
         img = blend_image(img)
     if palette and isinstance(palette, string_or_bytes):
         palette = palette.split()
-    return imageops.quantize(img, max_colors, dither, [QColor(x).rgb() for x in palette])
+    return imageops.quantize(img, int(max_colors), dither, tuple(QColor(x).rgb() for x in palette))
 
 
 def eink_dither_image(img):