Assigning value to returned shared_ptr doesn't behave as expected

I have a private three dimensional vector of shared_ptr<Room> objects as follows:

private:
    vector<vector<vector<shared_ptr<Room>>>> world;

In the same class, I provide access to Room objects:

public:
    shared_ptr<Room> room_at(const int & x, const int & y, const int & z) const
    {
        return world.at(x).at(y).at(z);
    }

Also in the same class, I initialize the world structure:

for (int x = 0; x < C::WORLD_X_DIMENSION; ++x)
{
    vector<vector<shared_ptr<Room>>> row;
    for (int y = 0; y < C::WORLD_Y_DIMENSION; ++y)
    {
        vector<shared_ptr<Room>> vertical_stack; // "stack"
        for (int z = 0; z < C::WORLD_Z_DIMENSION; ++z)
        {
            vertical_stack.push_back(shared_ptr<Room>(nullptr));
        }
        row.push_back(vertical_stack);
    }
    world.push_back(row);
}

Later, I want to save a Room object into world:

void add_room_to_world(const int & x, const int & y, const int & z)
{
    shared_ptr<Room> room = make_shared<Room>(); // create an empty room

    /* (populate room's member fields) */

    // add room to world
    room_at(x, y, z) = room; // This doesn't work as expected
}

The shared_ptr within world starts out as nullptr as expected, but doesn't change on the last line above.

Based on what I've found on SO, I've tried operator= (above), .reset(room), and make_shared<Room>(room) (using an actual Room object rather than shared_ptr<Room>) but in all cases, the shared_ptr within world stays set to nullptr.

What is the correct way to assign objects into world?