C++11 线程安全队列

A project I'm working on uses multiple threads to do work on a collection of files. Each thread can add files to the list of files to be processed, so I put together (what I thought was) a thread-safe queue. Relevant portions follow:

// qMutex is a std::mutex intended to guard the queue
// populatedNotifier is a std::condition_variable intended to
//                   notify waiting threads of a new item in the queue

void FileQueue::enqueue(std::string&& filename)
{
    std::lock_guard<std::mutex> lock(qMutex);
    q.push(std::move(filename));

    // Notify anyone waiting for additional files that more have arrived
    populatedNotifier.notify_one();
}

std::string FileQueue::dequeue(const std::chrono::milliseconds& timeout)
{
    std::unique_lock<std::mutex> lock(qMutex);
    if (q.empty()) {
        if (populatedNotifier.wait_for(lock, timeout) == std::cv_status::no_timeout) {
            std::string ret = q.front();
            q.pop();
            return ret;
        }
        else {
            return std::string();
        }
    }
    else {
        std::string ret = q.front();
        q.pop();
        return ret;
    }
}

但是，我偶尔会在 if (...wait_for(lock, ti??meout) == std::cv_status::no_timeout) { } 块中出现段错误，并且在 gdb 中检查表明由于队列为空，出现段错误.这怎么可能?我的理解是 wait_for 只在收到通知时返回 cv_status::no_timeout，并且这应该只在 FileQueue::enqueue 之后发生刚刚将一个新项目推送到队列中.