diff --git a/doc/examples/plot_adapt_rgb.py b/doc/examples/plot_adapt_rgb.py
index 330e0e2e..35b86d29 100644
--- a/doc/examples/plot_adapt_rgb.py
+++ b/doc/examples/plot_adapt_rgb.py
@@ -91,7 +91,7 @@ def sobel_gray(image):
     return filter.sobel(image)
 
 fig, ax = plt.subplots()
-ax.imshow(sobel_gray(image))
+ax.imshow(sobel_gray(image), cmap=plt.cm.gray)
 
 plt.show()