fixed bug where tensor.grad tried to change tuples

theano/gradient.py

@@ -547,8 +547,10 @@ def grad(cost, wrt, g_cost = None, consider_constant = None, warn_type = 'ignore
         #populate term_dict[node] and return it
         def access_term_cache(node):
             if node not in term_dict:
-                term_dict[node] = node.op.grad(node.inputs,
-                        [access_grad_cache(var) for var in node.outputs])
+                #must convert to list in case the op returns a tuple
+                #we won't be able to post-process out the Nones if it does that
+                term_dict[node] = list(node.op.grad(node.inputs,
+                        [access_grad_cache(var) for var in node.outputs]))
                 for i in xrange(len(term_dict[node])):
                     if term_dict[node][i] is None:
                         term_dict[node][i] = tensor.zeros_like(node.inputs[i])