diff --git a/image-processing-with-numpy.ipynb b/image-processing-with-numpy.ipynb
index 1da3908..a31eaff 100644
--- a/image-processing-with-numpy.ipynb
+++ b/image-processing-with-numpy.ipynb
@@ -293,7 +293,7 @@
    },
    "outputs": [],
    "source": [
-    "def to_grayscale(im, weights = np.c_[0.2989, 0.5870, 0.1140]):\n",
+    "def to_grayscale(im, weights = np.c_[-0.2989, -0.5870, -0.1140]):\n",
     "    \"\"\"\n",
     "    Transforms a colour image to a greyscale image by\n",
     "    taking the mean of the RGB values, weighted\n",