Python: TypeError: Can't convert 'generator' object to str implicitly

I'm doing an assignment and here is what the class looks like:

class GameStateNode:
    '''
    A tree of possible states for a two-player, sequential move, zero-sum,
    perfect-information game.

    value: GameState -- the game state at the root of this tree
    children: list -- all possible game states that can be reached from this
    game state via one legal move in the game.  children is None until grow
    is called.
    '''

    def __init__(self, game_state):
        ''' (GameStateNode, GameState) -> NoneType

        Initialize a new game state tree consisting of a single root node 
        that contains game_state.
        '''
        self.value = game_state
        self.children = []

I then wrote these two functions in because I need a recursive str:

    def __str__(self):
        ''' (GameStateNode) -> str '''    
        return _str(self)

def _str(node):
    ''' (GameStateNode, str) -> str '''
    return ((str(node.value) + '\n') + 
            ((str(child) for child in node.children) if node.children else ''))

Can somebody tell me what the problem is with my _str function?