Argument must be an instance of Identifier mean....?

when I use follow code :

IQueryableAttribute attribute_001= findAttribute(projectArea, auditableClient, "com.ibm.team.workitem.workItemType.requirement.riskAssessment", null);

			AttributeExpression attrExpression_001= new AttributeExpression(attribute_001, AttributeOperation.CONTAINS,"");

			List list_workItem = queryClient.getGroupCounts(projectArea, attrExpression_001, "hello", null);

			logger.info(list_workItem.size());

case the follow exception :

Exception in thread "main" java.lang.IllegalArgumentException: Argument must be an instance of Identifier
at com.ibm.team.workitem.common.internal.model.EnumerationAttributeType.toString(EnumerationAttributeType.java:81)
at com.ibm.team.workitem.common.internal.expression.AttributeValueFactory$ConstantValue.saveState(AttributeValueFactory.java:58)
at com.ibm.team.workitem.common.expression.AttributeExpression.saveValueProxy(AttributeExpression.java:165)
at com.ibm.team.workitem.common.expression.AttributeExpression.saveState(AttributeExpression.java:155)
at com.ibm.team.workitem.common.internal.expression.XMLExpressionSerializer.serialize(XMLExpressionSerializer.java:132)
at com.ibm.team.workitem.common.internal.expression.XMLExpressionSerializer.serialize(XMLExpressionSerializer.java:57)
at com.ibm.team.workitem.common.internal.query.impl.QueryDescriptorCustomImpl.setExpression(QueryDescriptorCustomImpl.java:40)
at com.ibm.team.workitem.common.internal.query.QueryCommon.createQuery(QueryCommon.java:258)
at com.ibm.team.workitem.common.internal.query.QueryCommon.getGroupCounts(QueryCommon.java:239)
at com.horizon.rqm.FindWorkItemsByAttributes.main(FindWorkItemsByAttributes.java:103)

can someone tell the reason ? what should I do?