We upgraded a visual studio project from Data objects v3.7 to Xtensive v5.0.6. When I build the domain against the database that was configured with v3.7, I get an error that says

An exception of type 'System.NotSupportedException' occurred in Xtensive.Orm.dll but was not handled in user code.

‘Type 'Unknown' is not supported.’ What does this error mean?

This is what my sql.log file shows:

2015-09-18 15:05:30.9907 | Info | Xtensive.Orm.Sql | Session 'n/a'. Creating connection. 
2015-09-18 15:05:31.0077 | Info | Xtensive.Orm.Sql | Session 'n/a'. Opening connection 'sqlserver://localhost\SQLSERVER2008/SeasiteDO'. 
2015-09-18 15:05:31.0077 | Info | Xtensive.Orm.Sql | Session 'n/a'. Beginning transaction @ . 
2015-09-18 15:05:31.5907 | Info | Xtensive.Orm.Sql | Session 'n/a'. SQL batch: 
SELECT [a].[Name] AS [Name], [a].[Version] AS [Version] FROM [dbo].[Metadata.Assembly] [a] 
2015-09-18 15:35:11.5292 | Info | Xtensive.Orm.Sql | Session 'n/a'. Creating connection. 
2015-09-18 15:35:11.5522 | Info | Xtensive.Orm.Sql | Session 'n/a'. Opening connection 'sqlserver://localhost\SQLSERVER2008/SeasiteDO'. 
2015-09-18 15:35:11.5662 | Info | Xtensive.Orm.Sql | Session 'n/a'. Beginning transaction @ . 
2015-09-18 15:35:12.2812 | Info | Xtensive.Orm.Sql | Session 'n/a'. SQL batch: 
SELECT [a].[Name] AS [Name], [a].[Version] AS [Version] FROM [dbo].[Metadata.Assembly] [a]

I am only trying to build a single (newly upgraded) entity against our old database. I am using the PerformSafely configuration when building the domain.

*EDIT 9/21/2015*

at Xtensive.Sql.TypeMappingRegistry.MapSqlType(SqlType sqlType)
   at Xtensive.Orm.Providers.StorageDriver.MapSqlType(SqlType sqlType)
   at Xtensive.Orm.Upgrade.SqlModelConverter.ExtractType(TableColumn column)
   at Xtensive.Orm.Upgrade.SqlModelConverter.VisitTableColumn(TableColumn tableColumn)
   at Xtensive.Sql.Model.SqlModelVisitor`1.Visit(Node node)
   at Xtensive.Orm.Upgrade.SqlModelConverter.Visit(Node node)
   at Xtensive.Orm.Upgrade.SqlModelConverter.VisitTable(Table table)
   at Xtensive.Sql.Model.SqlModelVisitor`1.Visit(Node node)
   at Xtensive.Orm.Upgrade.SqlModelConverter.Visit(Node node)
   at Xtensive.Orm.Upgrade.SqlModelConverter.VisitSchema(Schema schema)
   at Xtensive.Orm.Upgrade.SqlModelConverter.VisitCatalog(Catalog catalog)
   at Xtensive.Orm.Upgrade.SqlModelConverter.Run()
   at Xtensive.Orm.Upgrade.SchemaExtractor.GetSchema()
   at Xtensive.Orm.Upgrade.UpgradingDomainBuilder.SynchronizeSchema(Domain domain, SchemaUpgrader upgrader, SchemaExtractor extractor, SchemaUpgradeMode schemaUpgradeMode)
   at Xtensive.Orm.Upgrade.UpgradingDomainBuilder.PerformUpgrade(Domain domain, UpgradeStage stage)
   at Xtensive.Orm.Upgrade.UpgradingDomainBuilder.BuildMultistageDomain()
   at Xtensive.Orm.Upgrade.UpgradingDomainBuilder.Run()
   at Xtensive.Orm.Upgrade.UpgradingDomainBuilder.Build(DomainConfiguration configuration)
   at Xtensive.Orm.Domain.Build(DomainConfiguration configuration)
   at Initializer.Initialize() in c:\Users\####\Documents\Visual Studio 2013\WebSites\WebSite5\App_Code\Initializer.cs:line 26
   at ASP.global_asax.Application_Start(Object sender, EventArgs e) in c:\Users\####\Documents\Visual Studio 2013\WebSites\WebSite5\Global.asax:line 8