diff --git a/8- Training a neural network: Implementing back propagation from scratch/code/mlp.py b/8- Training a neural network: Implementing back propagation from scratch/code/mlp.py
index d3f323a..5c91076 100644
--- a/8- Training a neural network: Implementing back propagation from scratch/code/mlp.py	
+++ b/8- Training a neural network: Implementing back propagation from scratch/code/mlp.py	
@@ -176,7 +176,7 @@ def _sigmoid_derivative(self, x):
         Returns:
             y (float): Output
         """
-        return x * (1.0 - x)
+        return self._sigmoid(x) * (1.0 - self._sigmoid(x))
 
 
     def _mse(self, target, output):