diff --git a/keras/layers/advanced_activations.py b/keras/layers/advanced_activations.py
index 151526bd8..9471defd3 100644
--- a/keras/layers/advanced_activations.py
+++ b/keras/layers/advanced_activations.py
@@ -74,7 +74,7 @@ class ParametricSoftplus(MaskedLayer):
         self.alphas = sharedX(self.alpha_init * np.ones(input_shape))
         self.betas = sharedX(self.beta_init * np.ones(input_shape))
         self.params = [self.alphas, self.betas]
-        self.input_shape = input_shape
+        self._input_shape = input_shape
 
         if self.initial_weights is not None:
             self.set_weights(self.initial_weights)