将位置分配给SKEmitterNode时对成员错误的不明确引用

Question

我正在尝试将CGPoint位置分配给SKEmitterNode

// Add the explosive emitter
var pos = CGPointMake(gs.frame.size.height / 2, gs.frame.size.width / 2);


var fireEmitterPath = NSBundle.mainBundle().pathForResource("Fire", ofType: "sks");
var fireEmitter = NSKeyedUnarchiver.unarchiveObjectWithFile(fireEmitterPath!);
fireEmitter?.position = pos; // ERROR HERE 
fireEmitter?.runAction(SKAction.sequence([SKAction.waitForDuration(0.2), SKAction.scaleBy(1.5, duration: 0.1), SKAction.runBlock({ 
    fireEmitter?.particleBirthRate = 0;
})]))

// add the smoke emitter
var smokeEmitterPath = NSBundle.mainBundle().pathForResource("Smoke", ofType: "sks");
var smokeEmitter = NSKeyedUnarchiver.unarchiveObjectWithFile(smokeEmitterPath!);
smokeEmitter?.position = pos;

gs.addChild(smokeEmitter);
smokeEmitter?.runAction(SKAction.sequence([SKAction.waitForDuration(0.2), SKAction.scaleBy(1.2, duration: 0.2), SKAction.runBlock({ 
    smokeEmitter?.particleBirthRate = 0;
})]))

分配emitter.position时，我收到编译错误：

  

对成员＆＃39;位置＆＃39;

的模棱两可的提及

任何意见都赞赏。