json：无法将对象解组为Auction.Item类型的Go值

I have a problem deserializing my object. I use an interface to this object to call the Serialization, and from reading the output the serialization works perfectly. Here is the underlying struct of my object:

type pimp struct {
    Price       int
    ExpDate     int64
    BidItem     Item
    CurrentBid  int
    PrevBidders []string
}

And here's the interface it implements:

type Pimp interface {
    GetStartingPrice() int
    GetTimeLeft() int64
    GetItem() Item
    GetCurrentBid() int
    SetCurrentBid(int)
    GetPrevBidders() []string
    AddBidder(string) error
    Serialize() ([]byte, error)
}

The Serialize() method:

func (p *pimp) Serialize() ([]byte, error) {
    return json.Marshal(*p)
}

As you may have noticed, pimp has a variable by the name of Item. This item is also, an interface:

type item struct {
    Name string
}

type Item interface {
    GetName() string
}

Now serializing a sample of such an object returns the following JSON:

{"Price":100,"ExpDate":1472571329,"BidItem":{"Name":"ExampleItem"},"CurrentBid":100,"PrevBidders":[]}

Here is my deserialization code:

func PimpFromJSON(content []byte) (Pimp, error) {
    p := new(pimp)
    err := json.Unmarshal(content, p)
    return p, err
}

Running it, however, gives me the following error:

json: cannot unmarshal object into Go value of type Auction.Item

Any help is appreciated.

The unmarshaler does not know the concrete type to use for the nil BidItem field. You can fix this by setting the field to a value of the appropriate type:

func PimpFromJSON(content []byte) (Pimp, error) {
    p := new(pimp)
    p.BidItem = &item{}
    err := json.Unmarshal(content, p)
    return p, err
}

playground example