template inheritance - protected base member undeclared

Question

Assuming the following example:

#include <iostream>
#include <mutex>

template< typename S >
class Base
{
public:
    Base () { std::cout << "hello" << std::endl; }
    virtual S get () { return S(5); }
protected:
    std::mutex mtx;
};

template< typename S >
class Derived : public Base< S >
{
public:
    using Base< S >::Base;
    virtual void lock () { std::unique_lock<std::mutex> lck(mtx); } // compilation error
};

int main ()
{
    Derived< int > d; // hello
    std::cout << d.get() << std::endl; // 5
    d.lock();

    return 0;
}

Why g++ compiler "can't see" member variable mtx ??

$ g++ -Wall -g -std=c++11 -o inher main.cpp
main.cpp: In member function ‘virtual void Derived<S>::lock()’:
main.cpp:19:65: error: ‘mtx’ was not declared in this scope
     virtual void lock () { std::unique_lock<std::mutex> lck(mtx); }
                                                             ^

Even if mtx is declared protected in Base using public inheritance? According to http://www.learncpp.com/cpp-tutorial/115-inheritance-and-access-specifiers/ and others, derived class has access to protected Base members.

Is it some template issue? How can I access protected members of Base class? Thanks for hints.

Note: example does not work even if I use int instead of std::mutex