TerrainData.GetHeights NEED HELP

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);
     }
 }

}