slicing: slicing now thorughs the right error

GPy/kern/_src/add.py

@@ -167,4 +167,10 @@ class Add(CombinationKernel):
         else:
             self.add_parameter(other)
         self.input_dim, self.active_dims = self.get_input_dim_active_dims(self.parts)
-        return self
+        return self
+
+    def input_sensitivity(self):
+        in_sen = np.zeros(self.input_dim)
+        for i, p in enumerate(self.parts):
+            in_sen[p.active_dims] += p.input_sensitivity()
+        return in_sen

GPy/kern/_src/kernel_slice_operations.py

@@ -42,6 +42,7 @@ class _Slice_wrap(object):
             self.X2 = self.k._slice_X(X2) if X2 is not None else X2
             self.ret = True
         else:
+            assert X.shape[1] == self.k.input_dim, "You did not specify active_dims and X has wrong shape: X_dim={} -- input_dim={}".format(X.shape[1], self.input_dim)
             self.X = X
             self.X2 = X2
             self.ret = False