         }
         
         internal func existingHash(for instance: BaseModel) -> Int? {
-            return storage[instance._id]?.hash
+            return objectPoolMutationQueue.sync {
+                return storage[instance._id]?.hash
+            }
         }
         
         internal func updateHash(for instance: BaseModel, with newHash: Int?) {

        Meow.log("Init")
        Meow.database = db
        Meow.types = types
        Meow.pool = ObjectPool()
        
        scheduleMaintenance()
    }

    }
    
    /// The Object Pool instance. For more information, look at the `ObjectPool` documentation.
    public static var pool = ObjectPool()
    
    internal static let maintenanceQueue = DispatchQueue(label: "org.openkitten.meow.maintenance", qos: .background)
    
