Resources.LoadAll Failing When Not Generic Object Class

I have some editor scripts where I am loading all assets at a path. So I am running this code not during play mode.

The following code works:

 public void LoadAllCards()
 {
     Object[] assets = Resources.LoadAll("ScriptableObjects/Cards", typeof(Object)) as Object[];
     Debug.Log(assets);
     foreach (Object card in assets)
     {
         Debug.Log(card);
     }
 }

The following code does NOT work. assets is NULL always in this case. M_Card is just a ScriptableObject.

 public void LoadAllCards()
 {
     cards = new List<M_Card>();
     M_Card[] assets = Resources.LoadAll("ScriptableObjects/Cards", typeof(M_Card)) as M_Card[];
     Debug.Log(assets);
     foreach (M_Card card in assets)
     {
         Debug.Log(card);
     }
 }

Do you know why Resources.LoadAll is working just fine if I use generic objects (and when I later cast it to M_Card, it works just fine), but is not working when I specify a specific class (like ScriptableObject or M_Card)?