321 Countdown Coroutine stuck on 3

So im trying to make a Coroutine that will count down from 3 whilst the Time.timeScale is set to 0 but it only ever displays the 3 then if Time.timeScale is still zero it freezes and if Time.timeScale is 1 then it displays the 3 then the game resumes any idea why this might be?

Coroutine Calling in the update method :

 if (counting) {
             Time.timeScale =0f;
             score = 0; 
             scoreText.text = "Score: 0";
             StartCoroutine (GameCountdown());
             Time.timeScale = 1f;
             counting = false;
         }
 

Coroutine:

     IEnumerator GameCountdown()
     {
 
         countdownText.text = "3";
         yield return new WaitForSeconds (1f);
         countdownText.text = "2";
         yield return new WaitForSeconds (1f);
         countdownText.text = "1";
         yield return new WaitForSeconds (1f);
         countdownText.text = "GO!";
 
 
     }