diff --git a/coal-crafter.lua b/coal-crafter.lua
index 0267bb7..a7d586c 100644
--- a/coal-crafter.lua
+++ b/coal-crafter.lua
@@ -19,16 +19,26 @@ local function waitForCoalEssence(inventory)
   end
 end
 
+local function strictTransferOne(slot)
+  local ok = turtle.transferTo(slot, 1)
+
+  if not ok then
+    error('cannot transfer item')
+  end
+
+  return ok
+end
+
 local function prepareCraftShape()
-  turtle.transferTo(2, 1)
-  turtle.transferTo(3, 1)
+  strictTransferOne(2)
+  strictTransferOne(3)
 
-  turtle.transferTo(5, 1)
-  turtle.transferTo(7, 1)
+  strictTransferOne(5)
+  strictTransferOne(7)
 
-  turtle.transferTo(9, 1)
-  turtle.transferTo(10, 1)
-  turtle.transferTo(11, 1)
+  strictTransferOne(9)
+  strictTransferOne(10)
+  strictTransferOne(11)
 end
 
 local function craft()