Hibernate version:
NHibernate 2.0.1.GA

I have got exception on SchemaUpdate:
System.InvalidCastException: Specified cast is not valid.
at NHibernate.Tool.hbm2ddl.ColumnMetadata..ctor(DataRow rs)
at NHibernate.Tool.hbm2ddl.TableMetadata.AddColumn(DataRow rs)
at NHibernate.Tool.hbm2ddl.TableMetadata.InitColumns(ISchemaReader meta)
at NHibernate.Tool.hbm2ddl.TableMetadata..ctor(DataRow rs, ISchemaReader meta, Boolean extras)
at NHibernate.Tool.hbm2ddl.DatabaseMetadata.GetTableMetadata(String name, String schema)
at NHibernate.Cfg.Configuration.GenerateSchemaUpdateScript(Dialect dialect, DatabaseMetadata databaseMetadata)
at NHibernate.Tool.hbm2ddl.SchemaUpdate.Execute(Boolean script, Boolean doUpdate)

SchemaExport works fine.

My config:
<property name="connection.driver_class">NHibernate.Driver.MySqlDataDriver</property>
<property name="dialect">NHibernate.Dialect.MySQL5Dialect</property>

Name and version of the database you are using:
MySQL 5.1.31