Count selected item in List

Hello, I want to simply check number of selected item, in my case, of empty Item in inventory. I created enums for items and basic constructor to create them without parameters which set type to "NULL". This how I initialize ALL inventory slots. I am using this also when player remove item from inventory, instead of deleting it from List I am simply rewriting it to "new Item()", thats why simple .Count() won't work for me.

For now I have simple method:

 int emptySlot = 0;
 foreach(Item item in inventoryItems)
     if(item.Type == Item.ItemType.NULL)
         emptySlot++;
 if(emptySlot == 0)
     return;

But I am wonder if there is way to do something like:

 if(inventoryItems.Count().Where(Item.ItemType.NULL) == 0)

Just to have less code =)