C++ Thread Local Singleton intermittent failure

I've tried to implement a very basic Thread Local Singleton class in C++ - it's a template class that other classes then inherit from. The problem is that it almost always works, but every now and again (say, 1 run in 15), it will fail with an error along the lines of:

* glibc detected * ./myExe: free(): invalid next size (fast): 0x00002b61a40008c0 ***

please forgive the rather contrived example below, but it serves to demonstrate the problem.

#include <thread>
#include <atomic>
#include <iostream>
#include <memory>
#include <vector>

using namespace std;

template<class T>
class ThreadLocalSingleton
{
public:
    /// Return a reference to an instance of the object
    static T& instance();

    typedef unique_ptr<T> UPtr;

protected:
    ThreadLocalSingleton() {}
    ThreadLocalSingleton(ThreadLocalSingleton const&);
    void operator=(ThreadLocalSingleton const&);
};

template<class T>
T& ThreadLocalSingleton<T>::instance()
{
    thread_local T m_instance;
    return m_instance;
}

// Create two atomic variables to keep track of the number of times the
// TLS class is created and accessed.
atomic<size_t> creationCount(0);
atomic<size_t> accessCount(0);

// Very simple class which derives from TLS
class MyClass : public ThreadLocalSingleton<MyClass>
{
    friend class ThreadLocalSingleton<MyClass>;
public:
    MyClass()
    {
        ++creationCount;
    }

    string getType() const
    {
        ++accessCount;
        return "MyClass";
    }
};

int main(int,char**)
{
    vector<thread> threads;
    vector<string> results;

    threads.emplace_back([&]() { results.emplace_back(MyClass::instance().getType()); MyClass::instance().getType(); });
    threads.emplace_back([&]() { results.emplace_back(MyClass::instance().getType()); MyClass::instance().getType(); });
    threads.emplace_back([&]() { results.emplace_back(MyClass::instance().getType()); MyClass::instance().getType(); });
    threads.emplace_back([&]() { results.emplace_back(MyClass::instance().getType()); MyClass::instance().getType(); });

    for (auto& t : threads)
    {
        t.join();
    }

    // Expecting 4 creations and 8 accesses.
    cout << "CreationCount: " << creationCount << " AccessCount: " << accessCount << endl;
}

I can replicate this on coliru, using the build command: g++ -std=c++11 -O2 -Wall -pedantic -pthread main.cpp && ./a.out

Many thanks!