Im Trying to Make a Terrian Paint Brush but i get a error

UnityException: Trying to access out-of-bounds terrain height information.

public class TerrianBrush : MonoBehaviour {

public Vector3 brushPostion;

public float[,] lastHight;

public Ray ray;
public RaycastHit hit;

// Use this for initialization
void Start () {
}

// Update is called once per frame
void Update () {
    ray = camera.ScreenPointToRay(Input.mousePosition);
    if (Physics.Raycast (ray,out hit)) {
       brushPostion = hit.point;
    }
    GameObject.Find("Brush").transform.position = brushPostion;
    if (Input.GetMouseButton(0)) {
       lastHight = Terrain.activeTerrain.terrainData.GetHeights(Mathf.RoundToInt(brushPostion.x), Mathf.RoundToInt(brushPostion.y),10,10);
    }
}

}