CRM 2011: plugin in sandbox mode works incorrectly with EntityCollection

Hi guys,

i have strange bug with plugin (only in sandbox mode):

1. load current entity from DB

2. mix attributes loaded entity and current entity in current entity

foreach (KeyValuePair<string, object> attr in secondaryEntity.Attributes)
            {string key = attr.Key;if (!mainEntity.Attributes.ContainsKey(key))
                {object value = attr.Value;//if (value is EntityCollection)
                        mainEntity.Attributes[key] = value;
                }
            }

after work of plugin the system generations an exception:

Unhandled Exception: System.ServiceModel.FaultException`1[[Microsoft.Xrm.Sdk.OrganizationServiceFault, Microsoft.Xrm.Sdk, Version=5.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35]]: System.Runtime.Serialization.SerializationException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #31550DA4Detail: <OrganizationServiceFault xmlns:i="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://schemas.microsoft.com/xrm/2011/Contracts"><ErrorCode>-2147220970</ErrorCode><ErrorDetails xmlns:d2p1="http://schemas.datacontract.org/2004/07/System.Collections.Generic" /><Message>System.Runtime.Serialization.SerializationException: Microsoft Dynamics CRM has experienced an error. Reference number for administrators or support: #31550DA4</Message><Timestamp>2011-10-10T09:25:54.6595704Z</Timestamp><InnerFault i:nil="true" /><TraceText>

[CDMBase.Plugins: CDMCRM11.Appointment_Update_Pre_SetPromotionStatus]
[32caaa30-e5e9-e011-9fb1-000c29b30c2d: Appointment_Update_Pre_SetPromotionStatus]


</TraceText></OrganizationServiceFault>

it only happens when there are fields of type 'EntityCollection' ( in 'appointment' case they are requiredattendees, optionalattendees, organizer)

What do you think about this?

Thanks in advance