[go-nuts] Why is this a runtime error (and not a compile error)?

package main

import "fmt"

type Animal interface {
Talk()
Move()
}

type Dog interface {
Animal
}

type Shepherd struct {
Dog
}

func New() Animal {
return &Shepherd{}
}

func (p *Shepherd) Talk() {
fmt.Println("woof!")
}

func main() {
fmt.Println("Hello, playground")

fido := New()

fido.Talk()
fido.Move()
}

woof!panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x1023e57]

goroutine 1 [running]:
main.(*Shepherd).Move(0xc010036160)
  /tmpfs/gosandbox-3b4927a9_89b3d8f1_41a3ab00_1dfeee01_401631ac/prog.go:1 +0x27
main.main()
  /tmpfs/gosandbox-3b4927a9_89b3d8f1_41a3ab00_1dfeee01_401631ac/prog.go:30 +0x85
  [process exited with non-zero status]


http://play.golang.org/p/m4ehAN2xFR


But, when I remove the Dog interface from the Shepherd struct (http://play.golang.org/p/BDfYULmjU4):


type Shepherd struct {

      // Dog

}


It gives me a compile error as expected:


prog.go:19: cannot use Shepherd literal (type *Shepherd) as type Animal in return argument:
  *Shepherd does not implement Animal (missing Move method)
  [process exited with non-zero status]



In other words: what exactly happens when I implement an interface in a struct? Is this a bug or expected behavior?