   }
 
   func testOptionalDouble() {
-    let userDefaults = UserDefaults.standard
+    let userDefaults = testDefaults!
     let key = "\(#function)\(#line)"
     XCTAssertFalse(userDefaults.hasKey(key))
 

  // TODO: set a boolean it's not working on Linux (Swift 4.1)
  // TODO: set nil to remove an object is not working on Linux (Swift 4.1)

  func testOptionalInteger() {
    let userDefaults = UserDefaults.standard
    let key = "\(#function)\(#line)"
    XCTAssertFalse(userDefaults.hasKey(key))


  }

  func testOptionalBool() {
    let userDefaults = testDefaults!
    let key = "\(#function)\(#line)"
    XCTAssertFalse(userDefaults.hasKey(key))

