nav mesh agent destination equal infinity

I try using nav mesh agent but something turn wrong

	void Update () 
	{

		float speed = 0f; 
		float angle = 0f; 

		nav.destination = player.transform.position;
		nav.speed = 5f;

		print (nav.destination);

	}

when i try to follow something but its turning wrong? Some one can help me? the result when i print out to console is (infinity,infinity,infinity);

might be because of this https://forum.unity3d.com/threads/navmesh-sampleposition-custom-area-bug.464369/