Reason needed - SearchException: Unable to guess FieldBridge for id

I create model Parameter,ParameterId.Parameter class contains primary key as ParameterId. ParameterId contains composite primary keys.I put indexing(@Indexed) for Parameter class but it throws SearchException. but i do not understand why it throws exception? because ParameterId contains composite primary key, so it cannot be indexed. but why Parameter class cannot be indexed. any one can you explain ?

thanks for help..