C++ map Access Violation while inserting. _Myhead is null

I have a map member in a class. The class is initialized well. When i try to add the first element to the map, the program crashes with Access Violation:

Exception thrown at 0x00007FFA9CCD538A (my.dll) in myprogram.exe: 0xC0000005: Access violation reading location 0x0000000000000000.

struct S {
    std::unordered_map<const A*, B*> mymap;
};

// Somewhere, S is created, accessible from somefunction.
S* s = new S();

somefunction(const A* a, B* b) 
{
    // s is accessible and not null.
    // s->mymap is not null
    // a is not null
    // b is not null    

    //s->mymap.emplace(a, b);   // fails
    //s->mymap[a] = b;  // also fails

    auto p = std::make_pair(a, b);
    s->mymap.insert(p); // also fails

}

I tried it with unordered_map as well as map. It seems some how the map is not initialized well. Why?