diff --git a/GPy/kern/_src/add.py b/GPy/kern/_src/add.py
index a5ca9a59..28250e06 100644
--- a/GPy/kern/_src/add.py
+++ b/GPy/kern/_src/add.py
@@ -46,7 +46,7 @@ class Add(Kern):
             [p.update_gradients_full(dL_dK, X[:,i_s], X2[:, i_s]) for p, i_s in zip(self._parameters_, self.input_slices)]
 
     def update_gradients_diag(self, dL_dKdiag, X):
-        [p.update_gradients_diag(dL_dK, X[:,i_s]) for p, i_s in zip(self._parameters_, self.input_slices)]
+        [p.update_gradients_diag(dL_dKdiag, X[:,i_s]) for p, i_s in zip(self._parameters_, self.input_slices)]
 
     def gradients_X(self, dL_dK, X, X2=None):
         """Compute the gradient of the objective function with respect to X.