             self.images[aKey] = nil
         }
 
-        dispatch_async(_queue, block)
+        dispatch_async(_set_queue, block)
     }
 }
 

    }
    let directory = Directory()

    private let _queue = dispatch_queue_create("swift.imageloader.queues.diskcached", DISPATCH_QUEUE_SERIAL)
}

// MARK: accessor

        }

        set {
            dispatch_barrier_async(_subscript_queue) {
                self.setObject(newValue!, forKey: aKey)
            }
        }
