title: TypeError: unsupported operand type
body: <p>Im getting the type error when attempting to check to see if the shipping variable was none (see below). the field is a currency field set using django... whats the correct way to check if its empty.</p>
  
  <pre><code>    Traceback (most recent call last):
  
      File "core/handlers/base.py", line 111, in get_response
      response = callback(request, *callback_args, **callback_kwargs)
  
  File "contrib/admin/options.py", line 307, in wrapper
  return self.admin_site.admin_view(view)(*args, **kwargs)
  
  File "utils/decorators.py", line 93, in _wrapped_view
  response = view_func(request, *args, **kwargs)
  
  File "views/decorators/cache.py", line 79, in _wrapped_view_func
  response = view_func(request, *args, **kwargs)
  
  File "contrib/admin/sites.py", line 197, in inner
  return view(request, *args, **kwargs)
  
  File "admin.py", line 21, in changelist_view
  return super(HeldOrderAdmin,self).changelist_view(request, extra_context=extra_context)
  
  File "utils/decorators.py", line 28, in _wrapper
  return bound_func(*args, **kwargs)
  
  File "utils/decorators.py", line 93, in _wrapped_view
  response = view_func(request, *args, **kwargs)
  
  File "utils/decorators.py", line 24, in bound_func
  return func(self, *args2, **kwargs2)
  
  File "contrib/admin/options.py", line 1179, in changelist_view
  ], context, context_instance=context_instance)
  
  File "shortcuts/__init__.py", line 20, in render_to_response
  return HttpResponse(loader.render_to_string(*args, **kwargs), **httpresponse_kwargs)
  
  File "template/loader.py", line 188, in render_to_string
  return t.render(context_instance)
  
  File "template/base.py", line 123, in render
  return self._render(context)
  
  File "template/base.py", line 117, in _render
  return self.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/loader_tags.py", line 127, in render
  return compiled_parent._render(context)
  
  File "template/base.py", line 117, in _render
  return self.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/loader_tags.py", line 127, in render
  return compiled_parent._render(context)
  
  File "template/base.py", line 117, in _render
  return self.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/loader_tags.py", line 127, in render
  return compiled_parent._render(context)
  
  File "template/base.py", line 117, in _render
  return self.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/loader_tags.py", line 64, in render
  result = block.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/loader_tags.py", line 64, in render
  result = block.nodelist.render(context)
  
  File "template/base.py", line 744, in render
  bits.append(self.render_node(node, context))
  
  File "template/base.py", line 757, in render_node
  return node.render(context)
  
  File "template/base.py", line 921, in render
  dict = func(*args)
  
  File "contrib/admin/templatetags/admin_list.py", line 232, in result_list
  'results': list(results(cl))}
  
  File "contrib/admin/templatetags/admin_list.py", line 217, in results
  yield ResultList(None, items_for_result(cl, res, None))
  
  File "contrib/admin/templatetags/admin_list.py", line 209, in __init__
  super(ResultList, self).__init__(*items)
  
  File "contrib/admin/templatetags/admin_list.py", line 137, in items_for_result
  f, attr, value = lookup_field(field_name, result, cl.model_admin)
  
  File "contrib/admin/util.py", line 214, in lookup_field
  value = attr(obj)
  
  File "admin.py", line 50, in total
  return '$%s' % (obj.order.total(),)
  
  File "models.py", line 234, in total
  if item.total is not None:
  
  File "models.py", line 356, in total
  shipping = 0
  
  TypeError: unsupported operand type(s) for +: 'NoneType' and 'Decimal'
  </code></pre>