Save on annotation entity fails with an exception in the event viewer.

I am trying to save the annotation entity in CRM 2011 using the following code :

<!-- language: c# -->

    var annotationContext = new CrmOrganizationServiceContext(crmOrganizationService);
    var annotation = annotationContext.CreateEntity("annotation");
    annotationContext.AddObject(annotation);
    // Setting the account id.
    entity.SetAttributeValue<EntityReference>("objectid", "account", "Guid of the account");    
    entity.iCrmSetPropertyValue(AnnotationDefinition.ColumnsName.DocumentBody,Convert.ToBase64String(new UnicodeEncoding().GetBytes("Sample Annotation Text")));
    entity.iCrmSetPropertyValue("isdocument",true);
    entity.iCrmSetPropertyValue(AnnotationDefinition.ColumnsName.Mime, "text/plain");
    entity.iCrmSetPickListValue(AnnotationDefinition.ColumnsName.ObjectType, 1);
    entity.iCrmSetPropertyValue(AnnotationDefinition.ColumnsName.Subject, "CompanyLogo");
    annotationContext.SaveChanges();

The save changes method fails with following error in the eventviewer of the crm server :

 The Web Service plug-in failed in OrganizationId: ###########;
 SdkMessageProcessingStepId: #####; EntityName: annotation; Stage: 30;
 MessageName: Create; AssemblyName:
 Microsoft.Crm.Extensibility.InternalOperationPlugin,
 Microsoft.Crm.ObjectModel, Version=5.0.0.0, Culture=neutral,
 PublicKeyToken=31bf3856ad364e35; ClassName:
 Microsoft.Crm.Extensibility.InternalOperationPlugin; Exception:
 Unhandled Exception: System.InvalidCastException: Specified cast is
 not valid.    at
 Microsoft.Crm.BusinessEntities.EntityNameReferenceAttributeConverter.ConvertToBusinessEntity(Object
 value, AttributeMetadata attributeMetadata, Dictionary`2
 abbrvToChildAttributeMetadata, ICrmConversionContext context,
 BusinessEntity target)    at
 Microsoft.Crm.BusinessEntities.EntityToBusinessEntityConverter.ConvertUsingExistingBusinessEntity(EntityMetadata
 entityMetadata, ICrmConversionContext conversionContext, Entity
 entity, BusinessEntity businessEntity)    at
 Microsoft.Crm.BusinessEntities.BusinessEntity.Converter.ConvertFrom(ITypeDescriptorContext
 context, CultureInfo culture, Object value)    at
 Microsoft.Crm.BusinessEntities.ConversionHelpers.Convert(ICrmConversionContext
 conversionContext, Object source, Type destinationType)    at
 Microsoft.Crm.Extensibility.DictionaryMapper.Map(ParameterCollection
 inputs, ICrmConversionContext context)    at
 Microsoft.Crm.Extensibility.InternalOperationPlugin.Execute(IServiceProvider
 serviceProvider)    at
 Microsoft.Crm.Extensibility.V5PluginProxyStep.ExecuteInternal(PipelineExecutionContext
 context)    at
 Microsoft.Crm.Extensibility.VersionedPluginProxyStepBase.Execute(PipelineExecutionContext
 context)