JSON marshall not working correctly when invoked via function parameter as interface

[urbim]

The following program sample.go triggers an unexpected result

package main

import (
    "encoding/json"
    "fmt"
)

type Foo interface {
}

type Foo1 struct{
    Bar1 string
}

func (f Foo1) MarshalJSON() ([]byte, error) {
    return []byte(`{"test": "foo"}`), nil
}

func main() {
    f1 := Foo1{
        Bar1: "bar1",
    }
    printJson(f1)
}

func printJson(f Foo) {
    marshal, err := json.Marshal(f)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(marshal))
}

Expected result

{"test":"foo"}

Got

null

Yaegi Version

v0.15.1

Additional Notes

Another sample with different result:

package main

import (
    "encoding/json"
    "fmt"
)

type Foo interface {
}

type Foo1 struct{
    Bar1 string
}

func (f Foo1) MarshalJSON() ([]byte, error) {
    return []byte(`{"test": "foo"}`), nil
}

type Foo2 struct{
    Bar2 string
    Foo1
}

func main() {
    f1 := Foo1{
        Bar1: "bar1",
    }
    f2 := Foo2{
        Bar2: "bar2",
        Foo1: f1,
    }
    printJson(f2)
}

func printJson(f Foo) {
    marshal, err := json.Marshal(f)
    if err != nil {
        panic(err)
    }
    fmt.Println(string(marshal))
}

Expected result

{"test":"foo"}

Got

{"Bar2":"bar2","Bar1":"bar1"}