diff --git a/09_unsupervised_learning.ipynb b/09_unsupervised_learning.ipynb
index b43ce51..68d8c5d 100644
--- a/09_unsupervised_learning.ipynb
+++ b/09_unsupervised_learning.ipynb
@@ -3070,7 +3070,7 @@
     "ss = np.linspace(1, 2, 101)\n",
     "XX, SS = np.meshgrid(xx, ss)\n",
     "ZZ = 2 * norm.pdf(XX - 1.0, 0, SS) + norm.pdf(XX + 4.0, 0, SS)\n",
-    "ZZ = ZZ / ZZ.sum(axis=1) / (xx[1] - xx[0])"
+    "ZZ = ZZ / ZZ.sum(axis=1)[:,np.newaxis] / (xx[1] - xx[0])"
    ]
   },
   {