Spring StopWatch concurrency

I have a Bean configured as follow:

    @Bean(name = "myStopWatch")
    @Scope(value = "prototype")
    public MyStopWatch myStopWatch() {
       return new MyStopWatch();
    }

And MyStopWatch class is as follow:

import org.springframework.util.StopWatch;

public class MyStopWatch {

   private StopWatch sw = new StopWatch();

   public void start() {
       if(!sw.isRunning()) {
           sw.start();
       }
   }

   public void stop() {
       if(sw.isRunning()) {
           sw.stop();
       }
   }
}

We're using this bean in a highly concurrent environment. If my understanding is correct, MyStopWatch class should never be shared bewtween thread, right?

However, we sometimes (very rarely) get the following error:

java.lang.IllegalStateException: Can't start StopWatch: it's already running at org.springframework.util.StopWatch.start(StopWatch.java:127) at org.springframework.util.StopWatch.start(StopWatch.java:116)

So far we couldn't reproduce this behavior with our test. I'm looking for more information regarding how I should correctly define my sw variable (or ma bean) in order to avoid this error.