Xcode 9.2 don't work without space

I have this code

import UIKit

class ViewController: UIViewController {

    @IBAction func touchCard(_ sender: UIButton) {
        flipCard(withEmoji: "  ", on: sender)
    }

    func flipCard(withEmoji emoji: String, on button: UIButton) {
        if button.currentTitle == emoji {
            button.setTitle("", for: UIControlState.normal)
            button.backgroundColor = #colorLiteral(red: 1, green: 0.5763723254, blue: 0, alpha: 1)
        } else {
            button.setTitle(emoji, for: UIControlState.normal)
            button.backgroundColor = #colorLiteral(red: 1, green: 1, blue: 1, alpha: 1)
        }
    }
}

The code must flip a card, but it don't work.

However, if I change this line of code and insert a space between the quotes, the code will work.

button.setTitle(" ", for: UIControlState.normal)

What happening? What is wrong? How to fix it?

Link to screenshot and video

https://drive.google.com/open?id=1LPjor519lRf6V-VBSQBMwoov_Q05GLSu

https://drive.google.com/open?id=1Wnm3SCEem2dZLJ9f4JRR_EfbZBDS-_0E