Searching for Orders Problem

Hi,

I need to find orders which has a special metaField and I am using EPIServer 8.11.0.0. I tried Order Search described in this  article. but it returns no row.

To make it simpler, I tried to find orders that the field is not null but still I there was no success. I know that I used the correct fields and classes since it returns data when I ask for "field is null". 

Any Guidance?

var searchOptions = new OrderSearchOptions
{
CacheResults = false,
StartingRecord = 0,
RecordsToRetrieve = 10,
Namespace = "Mediachase.Commerce.Orders"
};
OrderSearchParameters parameters = new OrderSearchParameters();
searchOptions.Classes.Add("OrderFormEx");
//parameters.SqlMetaWhereClause = string.Format( "META.DocNumber LIKE '%{0}%'",docNumber);
parameters.SqlMetaWhereClause = "NOT META.DocNumber IS NULL";
//parameters.SqlWhereClause = "OrderGroupId IN (Select ObjectId FROM OrderGroup_PurchaseOrder)";
var purchaseOrderCollection = OrderContext.Current.FindPurchaseOrders(parameters, searchOptions);

//Returns purchaseOrderCollection[0]