diff --git a/Notebooks/Chap15/15_2_Wasserstein_Distance.ipynb b/Notebooks/Chap15/15_2_Wasserstein_Distance.ipynb
index 332b36f..9384399 100644
--- a/Notebooks/Chap15/15_2_Wasserstein_Distance.ipynb
+++ b/Notebooks/Chap15/15_2_Wasserstein_Distance.ipynb
@@ -86,6 +86,7 @@
       "cell_type": "code",
       "source": [
         "# TODO Define the distance matrix from figure 15.8d\n",
+        "# The index should be normalized before being used in the distance calculation.\n",
         "# Replace this line\n",
         "dist_mat = np.zeros((10,10))\n",
         "\n",
@@ -243,4 +244,4 @@
       }
     }
   ]
-}
\ No newline at end of file
+}