类模板的公共继承意外失败

为什么下面的失败 error: 'a_' was not declared in this scope 在 Bar::get() ?

template <typename N>
class Foo
{
public:
  Foo() { }

protected:
  N a_;
};

template <typename N>
class Bar : public Foo<N>
{
public:
  Bar() : Foo<N>() { }

  N get() { return a_; }
};