diff --git a/GPy/kern/_src/add.py b/GPy/kern/_src/add.py
index cb73087e..7e707d30 100644
--- a/GPy/kern/_src/add.py
+++ b/GPy/kern/_src/add.py
@@ -172,7 +172,7 @@ class Add(CombinationKernel):
 
     def add(self, other, name='sum'):
         if isinstance(other, Add):
-            other_params = other._parameters_.copy()
+            other_params = other._parameters_[:]
             for p in other_params:
                 other.remove_parameter(p)
             self.add_parameters(*other_params)
diff --git a/GPy/testing/kernel_tests.py b/GPy/testing/kernel_tests.py
index 3eef6768..2b98183e 100644
--- a/GPy/testing/kernel_tests.py
+++ b/GPy/testing/kernel_tests.py
@@ -240,6 +240,7 @@ class KernelGradientTestsContinuous(unittest.TestCase):
 
     def test_Add(self):
         k = GPy.kern.Matern32(2, active_dims=[2,3]) + GPy.kern.RBF(2, active_dims=[0,4]) + GPy.kern.Linear(self.D)
+        k += GPy.kern.Matern32(2, active_dims=[2,3]) + GPy.kern.RBF(2, active_dims=[0,4]) + GPy.kern.Linear(self.D)
         k.randomize()
         self.assertTrue(check_kernel_gradient_functions(k, X=self.X, X2=self.X2, verbose=verbose))