Add assertion after code review

theano/gradient.py

@@ -795,6 +795,17 @@ def _populate_grad_dict(var_to_node_to_idx,
                     else:
                         new_output_grads.append(og)
 
+                # Make sure that, if new_output_grads[i] has a dtype:
+                # - it is the same dtype as outputs[i]
+                # - if the dtype is an int, then new_output_grads[i] is 0.
+                for o, ng in zip(node.outputs, new_output_grads):
+                    o_dt = getattr(o.type, 'dtype', None)
+                    ng_dt = getattr(ng.type, 'dtype', None)
+                    if ng_dt:
+                        assert ng_dt == o_dt
+                        if ng_dt in theano.tensor.discrete_dtypes:
+                            assert theano.get_constant_value(ng) == 0
+
                 input_grads = node.op.grad(inputs, new_output_grads)
 
                 if input_grads is None: