Bad code generated for shadowed struct fields

[nwidger]

If I create a package foo with the following contents:

package foo

type omit *struct{}

type Foo struct {
    NotHidden string `json:"not_hidden"`
    Hidden    string `json:"hidden"`
}

type ShortFoo struct {
    *Foo
    Hidden omit `json:"hidden,omitempty"`
}

and use it in the following main package:

package main

import (
    "encoding/json"
    "log"
)
import "github.com/nwidger/foo"

func main() {
    f := &foo.Foo{}
    sf := &foo.ShortFoo{Foo: f}
    buf, err := json.Marshal(sf)
    if err != nil {
        log.Fatal(err)
    }
    log.Println(string(buf))
}

I get the following output:

2015/09/02 15:51:52 {"not_hidden":""}

If I then run ffjson foo.go and rerun my main package I get the following build errors:

# github.com/nwidger/foo
../foo/foo_ffjson.go:289: cannot convert mj.Hidden (type omit) to type string
../foo/foo_ffjson.go:302: ffj_t_ShortFoo_Hidden redeclared in this block
    previous declaration at ../foo/foo_ffjson.go:298
../foo/foo_ffjson.go:309: ffj_key_ShortFoo_Hidden redeclared in this block
    previous declaration at ../foo/foo_ffjson.go:305
../foo/foo_ffjson.go:529: cannot use string(string(outBuf)) (type string) as type omit in assignment

Is behavior expected? I would assume since it works with the stdlib JSON encoder it should work with ffjson. I learned about the encapsulation/shadowing/omit trick from this blog post and often use it to restrict which fields are returned by my web app.