Go json.Unmarshal does not work with type definition of json.RawMessage

Question

I have a small Go program like this:

package main

import "encoding/json"

func main() {
    bs := []byte(`{"items": ["foo", "bar"]}`)

    data := Data{}
    err := json.Unmarshal(bs, &data)
    if err != nil {
        panic(err)
    }
}

type Data struct {
    Items []Raw `json:"items"`
}

// If I do this, this program panics with "illegal base64 data at input byte 0"
type Raw json.RawMessage

// This works
type Raw = json.RawMessage

Why does json.Unmarshal work with type alias but not type definition? What does that error message mean?