Message not being recieved

I have a gameobject, and two images: the inventory, the inventory slot, and the slot icon image. The inventory sends a message to the slot every update:

         item1 = inv[0];
         Slot1.SendMessage ("recieveContentData", item1);
         item2 = inv[1];
         Slot2.SendMessage ("recieveContentData", item2);
         item3 = inv[2];
         Slot3.SendMessage ("recieveContentData", item3);
         item4 = inv[3];
         Slot4.SendMessage ("recieveContentData", item4);

The slot receives this message:

     public void receiveContentData(Item newContents)
     {
         contents = newContents;
     }

And then, every update, tells the slot icon to change it's image to that of the item, contents.

         if (contents != null)
         {
             itemImage.enabled = true;
             itemImage.sprite = contents.itemsprite;
         }
         else
         {
             itemImage.sprite = null;
             itemImage.enabled = false;
         }
     }

The problem is, when I run this script, apparently there is no receiver to this message;
Even though I assigned all of the Slot variables to gameobjects with the slot script attached, and all of the slots have a Slot icon image as children. Here is the exact error message:

 SendMessage recieveContentData has no receiver!
 UnityEngine.GameObject:SendMessage(String, Object)