NHibernate.Mapping.Attributes.IdAttribute can not recognize Type of the member it's attached to?

[Alexey Pelykh]

This code generates exception that type was not specified:
[Id(0, Column = "_id")]
[Generator(1, Class = "native")]
public long Id { ... }

This does not:
[Id(0, Column = "_id", TypeType = typeof(long))]
[Generator(1, Class = "native")]
public long Id { ... }

Why? Properties determine their types quite fine..