diff --git a/src/block/WaterCauldron.php b/src/block/WaterCauldron.php
index 88327a010..fddccf43b 100644
--- a/src/block/WaterCauldron.php
+++ b/src/block/WaterCauldron.php
@@ -31,7 +31,6 @@ use pocketmine\item\Armor;
 use pocketmine\item\Banner;
 use pocketmine\item\Dye;
 use pocketmine\item\Item;
-use pocketmine\item\ItemBlock;
 use pocketmine\item\ItemTypeIds;
 use pocketmine\item\Potion;
 use pocketmine\item\PotionType;
@@ -155,7 +154,7 @@ final class WaterCauldron extends FillableCauldron{
 				$this->position->getWorld()->setBlock($this->position, $this->withFillLevel($this->getFillLevel() - self::CLEAN_BANNER_USE_AMOUNT));
 				$this->position->getWorld()->addSound($this->position->add(0.5, 0.5, 0.5), new CauldronCleanItemSound());
 			}
-		}elseif($item instanceof ItemBlock && $item->getBlock()->getTypeId() === BlockTypeIds::DYED_SHULKER_BOX){
+		}elseif(ItemTypeIds::toBlockTypeId($item->getTypeId()) === BlockTypeIds::DYED_SHULKER_BOX){
 			if($this->customWaterColor === null){
 				$newItem = VanillaBlocks::SHULKER_BOX()->asItem();
 				$newItem->setNamedTag($item->getNamedTag());
diff --git a/src/block/inventory/ShulkerBoxInventory.php b/src/block/inventory/ShulkerBoxInventory.php
index 102e0b648..d915a9951 100644
--- a/src/block/inventory/ShulkerBoxInventory.php
+++ b/src/block/inventory/ShulkerBoxInventory.php
@@ -26,7 +26,7 @@ namespace pocketmine\block\inventory;
 use pocketmine\block\BlockTypeIds;
 use pocketmine\inventory\SimpleInventory;
 use pocketmine\item\Item;
-use pocketmine\item\ItemBlock;
+use pocketmine\item\ItemTypeIds;
 use pocketmine\network\mcpe\protocol\BlockEventPacket;
 use pocketmine\network\mcpe\protocol\types\BlockPosition;
 use pocketmine\world\Position;
@@ -51,11 +51,9 @@ class ShulkerBoxInventory extends SimpleInventory implements BlockInventory{
 	}
 
 	public function canAddItem(Item $item) : bool{
-		if($item instanceof ItemBlock){
-			$blockTypeId = $item->getBlock()->getTypeId();
-			if($blockTypeId === BlockTypeIds::SHULKER_BOX || $blockTypeId === BlockTypeIds::DYED_SHULKER_BOX){
-				return false;
-			}
+		$blockTypeId = ItemTypeIds::toBlockTypeId($item->getTypeId());
+		if($blockTypeId === BlockTypeIds::SHULKER_BOX || $blockTypeId === BlockTypeIds::DYED_SHULKER_BOX){
+			return false;
 		}
 		return parent::canAddItem($item);
 	}
diff --git a/src/item/ItemIdentifier.php b/src/item/ItemIdentifier.php
index e0d0d28ba..8be4d7d56 100644
--- a/src/item/ItemIdentifier.php
+++ b/src/item/ItemIdentifier.php
@@ -31,9 +31,8 @@ class ItemIdentifier{
 	){}
 
 	public static function fromBlock(Block $block) : self{
-		//negative item type IDs are treated as block IDs
 		//TODO: maybe an ItemBlockIdentifier is in order?
-		return new self(-$block->getTypeId());
+		return new self(ItemTypeIds::fromBlockTypeId($block->getTypeId()));
 	}
 
 	public function getTypeId() : int{ return $this->typeId; }
diff --git a/src/item/ItemTypeIds.php b/src/item/ItemTypeIds.php
index b7eea8055..180842555 100644
--- a/src/item/ItemTypeIds.php
+++ b/src/item/ItemTypeIds.php
@@ -311,4 +311,19 @@ final class ItemTypeIds{
 	public static function newId() : int{
 		return self::$nextDynamicId++;
 	}
+
+	public static function fromBlockTypeId(int $blockTypeId) : int{
+		if($blockTypeId < 0){
+			throw new \InvalidArgumentException("Block type IDs cannot be negative");
+		}
+		//negative item type IDs are treated as block IDs
+		return -$blockTypeId;
+	}
+
+	public static function toBlockTypeId(int $itemTypeId) : int{
+		if($itemTypeId > 0){
+			throw new \InvalidArgumentException("Item type ID $itemTypeId does not represent a block");
+		}
+		return -$itemTypeId;
+	}
 }