diff --git a/15_autoencoders.ipynb b/15_autoencoders.ipynb
index 6114ce1..0140621 100644
--- a/15_autoencoders.ipynb
+++ b/15_autoencoders.ipynb
@@ -208,7 +208,7 @@
     "hidden = fully_connected(X, n_hidden, activation_fn=None)\n",
     "outputs = fully_connected(hidden, n_outputs, activation_fn=None)\n",
     "\n",
-    "mse = tf.reduce_sum(tf.square(outputs - X))\n",
+    "mse = tf.reduce_mean(tf.square(outputs - X))\n",
     "\n",
     "optimizer = tf.train.AdamOptimizer(learning_rate)\n",
     "training_op = optimizer.minimize(mse)\n",
@@ -1568,7 +1568,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.5.1"
+   "version": "3.5.2+"
   },
   "nav_menu": {
    "height": "381px",