Insert std::unique_ptr into a nested map

I am trying to make a game with a map of fields. My nested map looks like this

typedef std::map<unsigned int, std::map<unsigned int, std::unique_ptr<Field> Board;

Board is a member of my Game class which contains a lot of the logic. Basically I am trying to do this:

Board board_;
unsigned int x = 0;
unsigned int y = 0;
board_[x][y] = std::unique_ptr<Field>(new Wall(x, y));

But I get this error:

Game.cpp: In member function ‘int Game::addTileToBoard(unsigned int, unsigned int, char)’:
Game.cpp:103:51: error: expected primary-expression before ‘(’ token
 board_[x][y] = std::move(std::unique_ptr<Tile>(new Wall(x, y)));

OR:

Game.cpp:103:41: error: expected primary-expression before ‘(’ token
 board_[x][y] = std::unique_ptr<Field>(new Wall(x, y));

So how do I create an object and add a unique_ptr to it to a nested map? Also I am bound to use c++11 so make_unique is not an option :( any ideas? Ideally I do not want to use map::insert since this method should also be able to simply override existing values. Using the asignment operator should just do that but for some reason it just won't compile..

Thanks!