Unexpected error happenend

This is the rescue page. See the exception details below

ActiveRecordException

Message:
Could not perform FindByPrimaryKey for Style. Id: 

StackTrace:
   at Castle.ActiveRecord.ActiveRecordBase.FindByPrimaryKey(Type targetType, Object id, Boolean throwOnNotFound)
   at Castle.ActiveRecord.ActiveRecordMediator.FindByPrimaryKey(Type targetType, Object id, Boolean throwOnNotFound)
   at Castle.ActiveRecord.ActiveRecordMediator`1.FindByPrimaryKey(Object id, Boolean throwOnNotFound)
   at Ramtha.Core.Repositories.ProductRepository.getStyleById(String styleId) in D:\Web\QC-Ramtha\src\Ramtha.Core\Repositories\ProductRepository.cs:line 22
   at Ramtha.Controllers.ListController.Style(String style) in D:\Web\QC-Ramtha\Ramtha\Controllers\ListController.cs:line 112

Inner exception:
ArgumentNullException

Message:
null is not a valid identifier
Parameter name: id

StackTrace:
   at NHibernate.Impl.SessionImpl.Get(Type clazz, Object id)
   at Castle.ActiveRecord.ActiveRecordBase.FindByPrimaryKey(Type targetType, Object id, Boolean throwOnNotFound)