Is it not possible to use one-to-one mappings if the object being mapped (the target) uses a composite class as a primary key?

I am trying to create a one-to-one relation between a class, lets call it First, and another one lets call it second.

It wont allow me to do so if the class Second uses a @EmbeddedId as its primary key, how do i get around this?

Basically the structures are as follows:

@Entity
class First {
private Integer id;
private Second second;

@OneToOne
@JoinColumn(name="id", referencedColumnName="parentId", nullable=true)
public Second getSecond() {return second;}
... snip irrelevant methods ...
}

@Entity
class Second{
private Integer id;
private Integer parentId;
... snip irrelevant methods ...
}

The above works fine, but if i change it to

@Entity
class Second{
private MyCustomKey key;
private Integer parentId;
... snip irrelevant methods ...
}

I get a "Broken mapping" error.

I fail to see the difference, since both ids are manually assigned? Both the working version with a integer id, and the embedded id.