Multi-Object Editing not supported

I have just tried to script an editor script, and it just shows "multi-object editing not supported," and nothing else. the script:

 #pragma strict

@CustomEditor(PlayerController) @CanEditMultipleObjects

class PlayerEditor extends Editor{

 var Speed: float = 0.0;
 var SProp: SerializedProperty;
 
 function OnEnable(){
     SProp = serializedObject.FindProperty("MSpeed");
 }
 
 function OnInspectorGUI(){
     serializedObject.Update();
     Speed = EditorGUILayout.Slider(Speed, 1, 150);
     if(!SProp.hasMultipleDifferentValues){
         PBar(SProp / 100.0, Speed);
     }
     
 }
 
 function PBar(val: float, label: String){
     var rect: Rect = GUILayoutUtility.GetRect(18, 18, "Text Field");
     EditorGUI.ProgressBar(rect, val, label);
     EditorGUILayout.Space();
 }

}

is here. any help would be much appreciated. thanks in advance