initializer for struct with anonymous union.

[Danni Coy via Digitalmars-d-learn]

I currently have a linker error telling me

undefined reference to `initializer for event.Event`

Event is a struct with a union. Something Like

struct Event
{
    EventType type;
    union
    {
        EventA a;
        EventB b;
    }
}

Naming the union makes the linker errors go away.
Can anybody tell me what is going on here?