Enable physics on sprite

[merak]

Im working on simple platformer game and the piece of code below, I cannot activate physics to let walk my player with
game.physics.arcade.enable(player); 
whereever I put this, the error called Cannot read property 'enable' of null or Cannot read property 'arcade' of null appears
What exactly I need do to avoid this problem, thank you.

var game = new Phaser.Game(800, 600, Phaser.AUTO, '', {
  preload: preload,
  create: create,
  update: update },
  false, true, {
    default:'arcade',
    arcade:{
        gravity:{ y: 200}
    }



  });




function preload () {
	
this.game.load.image('stone' , "stone.png");
this.game.load.spritesheet('player', "Player/player_tilesheet.png", 80, 110,);

}

var player;
var controls = {};
var playerSpeed = 250;




function create () {

    this.game.stage.backgroundColor = 0x4888cc;
    this.player = game.add.sprite(360, 480 ,'player');
    this.player.animations.add('walk', [9,10],5,true);
    this.player.animations.play('walk');  
    
     controls = {
    	right: this.input.keyboard.addKey(Phaser.Keyboard.D),
    	left: this.input.keyboard.addKey(Phaser.Keyboard.A)


    };
}



function update () {

	if(controls.right.isDown){

		this.player.animations.play('walk');
		this.player.scale.setTo(1,1);
		this.playerSpeed += playerSpeed;


 if(controls.left.isDown){

    this.player.animations.play('walk');
    this.player.scale.setTo(-1,1);
    this.playerSpeed -= playerSpeed;

  
  }  

  }  

	}
}