     let source = PromiseSource<Int, NSError>()
     let pr = source.promise.dispatchOn(test1Queue)
 
-    pr
+    let _ = pr
       .mapError { error -> NSError in
         XCTAssertNotNil(DispatchQueue.getSpecific(key: test1QueueKey), "callback for queued dispatch method should be called on specified queue")
         XCTAssertEqual(error.code, 40)

    let source = PromiseSource<Int, NSError>()
    let pr = source.promise.dispatchOn(test1Queue)

    pr
      .flatMapError { error -> Promise<Int, NSError> in
        XCTAssertNotNil(DispatchQueue.getSpecific(key: test1QueueKey), "callback for queued dispatch method should be called on specified queue")
        XCTAssertEqual(error.code, 40)

    let source = PromiseSource<Int, NSError>()
    let pr = source.promise.dispatchOn(test1Queue)

    let _ = pr
      .flatMap { x -> Promise<Int, NSError> in
        XCTAssertNotNil(DispatchQueue.getSpecific(key: test1QueueKey), "callback for queued dispatch method should be called on specified queue")
        XCTAssertEqual(x, 40)
