Golang interface to bytes using gob encoder

interface_to_bytes.go

package main

import (
    "encoding/gob"
    "bytes"
)

func GetBytes(key interface{}) ([]byte, error) {
    var buf bytes.Buffer
    enc := gob.NewEncoder(&buf)
    err := enc.Encode(key)
    if err != nil {
        return nil, err
    }
    return buf.Bytes(), nil

}

This only works if the inputted interface was originally bytes. If this were to be a string, etc... you would receive something like this from a panic:

I solve this by converting the interface{} to string and then to []byte :

json.Unmarshal([]byte(value.(string)), &j)

This doesnt work also.
Unable to unmarshal: invalid character 'e' looking for beginning of value