Why does JBoss throw an "IllegalArgumentException: either callerSubject and callerRunAs should be non-null" sometimes when under a load?

Issue

• Why does JBoss throw an "IllegalArgumentException: either callerSubject and callerRunAs should be non-null" sometimes when under a load?

  2011-05-09 16:14:16,176 374983 ERROR [org.jboss.ejb.plugins.LogInterceptor] (WorkManager(2)-18:) RuntimeException in method: public abstract void org.example.jboss.SessionLocal.method(java.lang.String,java.lang.Integer):
  java.lang.IllegalArgumentException: Either callerSubject or callerRunAs should be non-null
    at org.jboss.security.plugins.javaee.EJBAuthorizationHelper.authorize(EJBAuthorizationHelper.java:78)
    at org.jboss.ejb3.security.RoleBasedAuthorizationInterceptorv2.invoke(RoleBasedAuthorizationInterceptorv2.java:189)
    at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:102)
    at org.jboss.ejb3.security.Ejb3AuthenticationInterceptorv2.invoke(Ejb3AuthenticationInterceptorv2.java:182)
    at org.jboss.aop.joinpoint.MethodInvocation.invokeNext(MethodInvocation.java:102)