tbb::concurrent_hash_map throws SIGSEGV

Question

I'm running a small program built using TBB on Windows with mingw32. It does a parallel_for. Inside the parallel_for my object makes changes to a concurrent_hash_map object. It starts running but later throws a SIGSEGV when I try to use an accessor. I don't know where the problem is.

My object:

class Foobar
{
public:
    Foobar(FoobarParent* rw) : _rw(rw)
    {
        _fooMap = &_rw->randomWalkers();
    }

    void operator() (const tbb::blocked_range<size_t>&r ) const
    {
        for(size_t i = r.begin(); i != r.end(); ++i)
        {
            apply(i);
        }
    }

private:
    void apply(int i) const
    {
        pointMap_t::accessor a;
        _fooMap->find(a, i);
        Point3D current = a->second;
        Point3D next = _rw->getNext(current);

        if (!_rw->hasConstraint(next))
        {
            return;
        }

        a->second = next;
    }

    FoobarParent* _rw;
    pointMap_t* _fooMap;
};

pointMap_t is defined as:

typedef tbb::concurrent_hash_map<int, Point3D> pointMap_t;

Can someone shed a light on this issue? I'm new to TBB. The signal is thrown when the apply method calls a->second.