fluent nhibernate HasOne WithForeignKey not working

Whenever I load a Task class, the Document property is always null, despite there being data in the db.

Task class:

public class Task
{
    public virtual Document Document { get; set; }

Task Mapping override for AutoPersistenceModel:

public void Override(AutoMap<Task> mapping)
{
    mapping.HasOne(x => x.Document)
        .WithForeignKey("Task_Id");

As you can see form what NHProf says is being run, the join condition is wrong, the WithForeignKey doesnt seem to take effect. In fact, i can write any string in the above code and it makes no difference.

FROM   [Task] this_
    left outer join [Document] document2_
    on this_.Id = document2_.Id

It should be:

FROM   [Task] this_
    left outer join [Document] document2_
    on this_.Id = document2_.Task_Id

If i hack the data in the db so that the ids match, then data is loaded, but obviously this is incorrect - but at least it proves it loads data.

Edit: rummaging in the fluent nhib source to find the XML produces this:

<one-to-one foreign-key="Task_Id" cascade="all" name="Document" class="MyProject.Document, MyProject, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" /> 

Edit: heres the schema:

CREATE TABLE [dbo].[Document](
[Id] [int] IDENTITY(1,1) NOT NULL,
[Task_Id] [int] NOT NULL,

CREATE TABLE [dbo].[Task](
[Id] [int] IDENTITY(1,1) NOT NULL,

Anyone got any ideas?

Thanks

Andrew