     let key = #function
     let transformerId = "transformer"
 
-    waitUntil { done in
+    waitUntil(timeout: 5) { done in
       cache.store(image, forKey: key) {
         cache.store(alternateImage, forKey: key, transformerId: transformerId) {
           cache.retrieveImage(forKey: key) { image, _ in

final class CacheTests: XCTestCase {
  
  private let helper = TestHelper()

  override func tearDown() {
    super.tearDown()

    Cache.default.clearMemory()
    Cache.default.clearDisk()
  }
  
  class MockStore: BackingStore {
    

    let image = helper.image
    let key = #function

    waitUntil(timeout: 5) { done in
      cache.store(image, forKey: key) {
        let urls = cache.expiredFileUrls()
        expect(urls).toNot(beEmpty())
