Always associate a mutex with its controlled data

Consider the following sketch of a thread-safe StreamingAverage class. There is a bug here; can you find it?

    class StreamingAverage {
      double m_sum = 0;
      int m_count = 0;
      double m_last_average = 0;
      std::mutex m_mtx;
    public:
      // Called from the single producer thread
      void add_value(double x) {
        std::lock_guard lk(m_mtx);
        m_sum += x;
        m_count += 1; // A
      }

      // Called from the single consumer thread
      double get_current_average() {
        std::lock_guard lk(m_mtx);
        m_last_average = m_sum / m_count; // B
        return m_last_average;
      }

      // Called from the single consumer thread
      double get_last_average() const {
        return m_last_average; // C
      }

      // Called from the single consumer thread
      double get_current_count() const {
        return...