Value cannot be null. Parameter name: item

Hi All,

We are using Dynamics CRM  and we try to retrieve values with the query.
Time to time we are receiving an error message like "Value cannot be null. Parameter name: item".
What we noticed is that this error occur either in context.OrganizationService.RetrieveMultiple(...) or context.OrganizationService.Retrieve(...) methods.
This methods are part of Microsoft.Xrm.Sdk.IOrganizationService:

Example of the code where the error occur: 

                    EntityCollection results = context.OrganizationService.RetrieveMultiple(
                        new QueryExpression
                        {
                            EntityName = Contact.EntityLogicalName,
                            ColumnSet = new ColumnSet(Contact.Fields.CustomField),
                            Criteria =
                            {
                                Filters =
                                {
                                    new FilterExpression
                                    {
                                        FilterOperator = LogicalOperator.And,
                                        Conditions =
                                        {
                                            new ConditionExpression(Contact.Fields.ContactId, ConditionOperator.Equal, contactId),
                                        }
                                    }
                                }
                            },
                        }
                    );