Problem using EditorGUILayout.ObjectField with custom type

So I have a class Skill, which is Serializable, and I want to add an inspector slot I can drag subclasses of Skill onto in order to customize those items in edit mode. My class currently reads:

using UnityEngine;
using System.Collections;

[System.Serializable]
public class Skill {
	public string name;
	public int id;
	public float castTime;
	public float cooldown;

	public Skill(string skillName, int skillID, float skillCastTime, float skillCooldown){
		name = skillName;
		id = skillID;
		castTime = skillCastTime;
		cooldown = skillCooldown;
	}


	public void Use(GameObject npc){

	}
}

I created a new variable of type Skill named itemSkill in my base item class, and in my database I’m trying to expose it with:

Skill itemSkill = EditorGUILayout.ObjectField(item.skillFunction, typeof(Skill), false);

However this generates an error, “The best overloaded match for `UnityEditor.EditorGUILayout.ObjectField(UnityEngine.Object, System.Type, params UnityEngine.GUILayoutOption)’ has some invalid arguments”. Is there an obvious syntax error, or am I not using EditorGUILayout properly?

for my editor I use this code:
public ItemManager itemManager;

itemManager = EditorGUILayout.ObjectField(itemManager, typeof(ItemManager)) as ItemManager;