c# - Define generator for composite-id property in NHibernate

Question

I've defined a composite-id for my NHibernate database model. It seems to work, but I cannot set a generator for one key-property.

Is it possible to define a generator for a key-propery in a composite-id?

C# NHibernate Model

public const string STR_ID = "Id";
public const string STR_SHARDKEY = "ShardKey";

[CompositeId(-4)]
[KeyProperty(-3, Name = STR_ID, Column = STR_ID)]
[KeyProperty(-2, Name = STR_SHARDKEY , Column = STR_SHARDKEY )]
[Generator(-1, Class = "guid.comb")]
public virtual Guid? Id { get; set; }

[Column(Name = STR_SHARDKEY )]
public virtual int ShardKey{ get; set; }

XML Mapping

<composite-id>
    <key-property name="Id" column="Id" />
    <key-property name="ShardKey" column="ShardKey" />
</composite-id>

Shouldn't there be a generator tag like <generator class="guid.comb" /> somewhere?