Making a player Invulnerable for a few seconds. Getting script errors that don't make sense.

The following is the script I’m using and I can’t for the life of me, figure out why it is not recognizing the ‘yield’ line.

Error: Assets/Scripts/PlayerDamage.cs(18,54): error CS1525: Unexpected symbol (', expecting )‘, ,', ;’, [', or =’

using UnityEngine;
using System.Collections;

public class PlayerDamage : MonoBehaviour {

	private bool  invulnerable = false;

	private void OnTriggerEnter2D(Collider2D collision)
	{

		if (!invulnerable) 
		{
			if (collision.gameObject.name == "Crowd(Clone)")
			{
			Debug.Log("Crowd Hit");
			GameObject.FindGameObjectWithTag("Player").GetComponent<PlayerHealth>().DamagePlayer(5);
				invulnerable = true;                    // pause damage
				yield WaitForSeconds (3) ;  // wait until unpausing damage
				invulnerable = false;                     // unpause damage 
			}
		}

	}
	
}

Problem’s Solved