unity editor scripting: toggle

I am working on unity editor for the first time, I have to add a simple toggle on the inspector, why does this code crash?

Code (CSharp):

1. public override void OnInspectorGUI()
2.         {
3.             GeometricPath path = target as GeometricPath;
4.  
5.             if (path.IsLerp)
6.             {
7.                 DrawDefaultInspector();
8.             }
9.             else
10.             {
11.                 EditorGUI.BeginChangeCheck();
12.                 bool isLerp = EditorGUILayout.Toggle("isLerp", path.IsLerp);
13.                 path.IsLerp = isLerp;
14.             }
15.         }
16. }