Why can't I cast Editor.target to my CustomEditor type?

I'm getting the error: "Cannot convert type UnityEngine.Object' to Mask'" when trying to cast Editor.target to my CustomEditor type. Why?

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 [UnityEditor.CustomEditor(typeof(Mask))]
 public class MaskEditor : UnityEditor.Editor {
 
     public override void OnInspectorGUI()
     {
         Mask mask = (Mask) target;
         // ... Draw something with EditorGUILayout static methods ...
     }
     
 }

Mask is a simple class which doesn't extends nothing and has a System.Serializable annotation.