diff --git a/neural_network/back_propagation_neural_network.py b/neural_network/back_propagation_neural_network.py
index 43e796e77be3..c358f6dd801b 100644
--- a/neural_network/back_propagation_neural_network.py
+++ b/neural_network/back_propagation_neural_network.py
@@ -21,8 +21,8 @@
 from matplotlib import pyplot as plt
 
 
-def sigmoid(x):
-    return 1 / (1 + np.exp(-1 * x))
+def sigmoid(x: np.ndarray) -> np.ndarray:
+    return 1 / (1 + np.exp(-x))
 
 
 class DenseLayer: