What if XCTestExpectation is unexpected

I'm writing an XCTest unit test in Swift. The idea is that a callback mustn't be called in a certain case.

So what I do, is

func testThatCallbackIsNotFired() {

    let expectation = expectationWithDescription("A callback is fired")
    // configure an async operation

    asyncOperation.run() { (_) -> () in
        expectation.fulfill()    // if this happens, the test must fail
    }

    waitForExpectationsWithTimeout(1) { (error: NSError?) -> Void in

        // here I expect error to be not nil,
        // which would signalize that expectation is not fulfilled,
        // which is what I expect, because callback mustn't be called
        XCTAssert(error != nil, "A callback mustn't be fired")
    }
}

When the callback is called, everything works fine: it fails with a message "A callback mustn't be fired" which is exactly what I need.

But if expectation hasn't been fulfilled, it fails and says

Asynchronous wait failed: Exceeded timeout of 1 seconds, with unfulfilled expectations: "Callback is fired".

Since a not fulfilled expectation is what I need, I don't want to have a failed test.

Do you have any suggestions what can I do to avoid this? Or, maybe, I can reach my goal in a different way? Thanks.