Why does this Around advice cause a StackOverflowError?

Why does this Around advice cause a StackOverflowError?

@Around("execution(* com.example.service.*.*(..))")
public Object monitor(ProceedingJoinPoint pjp) throws Throwable {
  long start = System.currentTimeMillis();
  Object result = monitor(pjp);
  long end = System.currentTimeMillis();
  System.out.println("Time: " + (end - start));
  return result;
}