具有嵌入式结构的Gorp插入

I am new to Gorp and golang.

https://github.com/go-gorp/gorp

I have a mysql table like

Table: product

Column: id(int), name(string), category1(string), category2(string)

and I created an embedded struct like

type (
    Product struct {
        Id       int
        Name     Names `db:"name"`
        Categories Categories

    }

    Categories struct {
        Category1 string `db:"category1"`
        Category2 string `db:"category2"`
    }
)

And I executed the following script

.......
product := &Product{-1,"foobar", Categories{"sample1", "sample2"}}
err := dbmap.Insert(product)
fmt.Println(err)

But I got this error message below

converting Exec argument $2 type: unsupported type main.Categories, a struct

I thought I had followed the manual of Struct Embedding.

https://github.com/go-gorp/gorp#struct-embedding

Please help me understand why I am still getting this error.