Using an Array of "MyClass" in EditorGUILayout.ObjectField ?

I'm having some trouble with this one!
First here's my Monobehavior class that's attached to my GameObject:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 [ExecuteInEditMode]
 public class ObjectAddOns : MonoBehaviour {
 
     private Transform t;
     public bool isObjectSlider;
 
     private ObjectSlider objectSlider;
 
 
     [System.Serializable]
     public class ObjectAddOn
     {
         public Transform addOn;
         public bool enabled;
         public ObjectAddOn( Transform addOn, bool enabled )
         {
             this.addOn = addOn;
             this.enabled = enabled;
         }
     }
     public ObjectAddOn[] objAddOns;
 
 
     #region Unity Functions
     // =================================== AWAKE FUNCTION =================================== //
 
     void Awake ()
     {
         if( !Application.isEditor )
             this.enabled = false;
 
         // Cache reference to the GameObject's Transform this script is attached to for speed
         t = this.transform;
 
         // Get the number of addOns attached to this GameObject
         int i = 1;
         foreach( Transform child in GetComponentsInChildren<Transform>() )
         {
             // Do NOT count the transform if it doesn't have a renderer attached
             if( child.renderer == null )
 
                 continue;
 
             
             // Create array of addons
             objAddOns = new ObjectAddOn[i];
             i++;
 
         }
 
     }
 
 
     // =================================== START FUNCTION =================================== //
     void Start ()
 
     {
         // Check if ObjectSlider class is already attached
         objectSlider = GetComponent<ObjectSlider>();
 
         // Populate addOns array with every addOn found on GameObject
         int i = 0;
         foreach( Transform child in GetComponentsInChildren<Transform>() )
         {
             if( child.renderer == null )
                 continue;
 
             objAddOns[i] = new ObjectAddOn( child, child.gameObject.active );
             i++;
         }
 
     }
 
 
     // =================================== UPDATE FUNCTION =================================== //
 
     void Update ()
     {
         // Add ObjectSlider class if enabled
         if( isObjectSlider && objectSlider == null )
         {
             gameObject.AddComponent<ObjectSlider>();
             objectSlider = GetComponent<ObjectSlider>();
         }
         else
         {
             if( objectSlider != null )
                 DestroyImmediate( objectSlider );
         }
 
         
         // Turn addons off or on (remove from Update for final build)
         for( int i = 0; i < objAddOns.Length; i++ )
         {
             objAddOns[i].addOn.gameObject.SetActiveRecursively( objAddOns[i].enabled );
         }
 
     }
     #endregion
 }

And here's the custom inspector class which extends it:

 using UnityEngine;
 using UnityEditor;
 using System.Collections;
 
 [CustomEditor( typeof( ObjectAddOns ))]
 public class ObjectAddOnsEditor : Editor {    
 
     private ObjectAddOns oa;
 
     
     #region Unity Functions
     // =================================== ONINSPECTORGUI FUNCTION =================================== //
     public override void OnInspectorGUI ()
     {
         oa = target as ObjectAddOns;
         for( int i = 0; i < oa.objAddOns.Length; i++ )
         {
             var t = oa.objAddOns[i];
             t = EditorGUILayout.ObjectField( t, typeof( ObjectAddOns.ObjectAddOn)) as ObjectAddOns.ObjectAddOn;
         }
     }
 }

The code above throws an error saying "Cannot convert "ObjectAddOns.ObjectAddOn" to "Object". How come EditorGUILayout.ObjectField will take types like Transform or Vector3 but not a custom type like ObjectAddons.ObjectAddon?

Any help would be very helpful (duh) :) Thanks for your time, as always! Stephane