This should work, but does not

should_work.go

package main

import (
	"bytes"
	"log"

	"github.com/linkedin/goavro"
)

var (
	schema = `
    {
        "name" : "event",
        "type" : "record",
        "namespace" : "mobi.bigtree.avro",
        "fields" : [
            { "type": "string", "name" : "string" },
            { "type": ["null", "string"], "name" : "nil_or_string" }
        ]
    }
    `
)

func main() {
	codec, err := goavro.NewCodec(schema)
	if err != nil {
		log.Fatalln("Failed to create codec.", err)
	}
	record, err := goavro.NewRecord(goavro.RecordSchema(schema))
	if err != nil {
		log.Fatalln("Failed to create record.", err)
	}
	record.Set("string", "this field cannnot be nil.")
	record.Set("nil_or_string", nil) // or a string
	buf := new(bytes.Buffer)
	if err = codec.Encode(buf, record); err != nil {
		log.Fatalln("Failed to encode.", err)
	}
	decode, err := codec.Decode(buf)
	if err != nil {
		log.Fatalln("Failed to decode.", err)
	}
	log.Println(decode)
}