diff --git a/minerva/pool.py b/minerva/pool.py
index a3e3683..a022836 100644
--- a/minerva/pool.py
+++ b/minerva/pool.py
@@ -59,5 +59,5 @@ class Pool:
 
 
     def cost(self):
-        return sum([mach.cost() for mach in machines])
+        return sum([mach.cost() for mach in self.machines])