How to implement interface method with return type is an interface in Golang

Here is my code:

type IA interface {
    FB() IB
}

type IB interface {
    Bar() string
}

type A struct {
    b *B
}

func (a *A) FB() *B {
    return a.b
}

type B struct{}

func (b *B) Bar() string {
    return "Bar!"
}

I get an error:

cannot use a (type *A) as type IA in function argument:
    *A does not implement IA (wrong type for FB method)
        have FB() *B
        want FB() IB

Here is the full code: http://play.golang.org/p/udhsZgW3W2
I should edit the IA interface or modifi my A struct?
What if I define IA, IB in a other package (so I can share these interface), I must import my package and use the IB as returned type of A.FB(), is it right?