passing xml with go, having multiple elements in lower case

I don't know how to make this code work. I'm just trying to parse a simple xml like this:

package main

import (
    "encoding/xml"
    "fmt"
)

type Data struct {
    XMLName xml.Name `xml:"data"`
    Nam     string   `xml:"nam,attr"`
}

type Struct struct {
    XMLName xml.Name `xml:"struct"`
    Data    []Data
}

func main() {

    x := `  
        <struct>
            <data nam="MESSAGE_TYPE">     
            </data>
            <data nam="MESSAGE_TYPE2">
            </data>
        </struct>
    `
    s := Struct{}
    err := xml.Unmarshal([]byte(x), &s)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v\n", s)
    fmt.Println(s.Data)
}

what I got is:

{{ struct} []}
[]

But when I change the "data" elements to Upper case, like:

package main

import (
    "encoding/xml"
    "fmt"
)

type Data struct {
    XMLName xml.Name `xml:"Data"`
    Nam     string   `xml:"nam,attr"`
}

type Struct struct {
    XMLName xml.Name `xml:"struct"`
    Data    []Data
}

func main() {

    x := `  
        <struct>
            <Data nam="MESSAGE_TYPE">     
            </Data>
            <Data nam="MESSAGE_TYPE2">
            </Data>
        </struct>
    `
    s := Struct{}
    err := xml.Unmarshal([]byte(x), &s)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%v\n", s)
    fmt.Println(s.Data)
}

I got this:

{{ struct} [{{ Data} MESSAGE_TYPE} {{ Data} MESSAGE_TYPE2}]}
[{{ Data} MESSAGE_TYPE} {{ Data} MESSAGE_TYPE2}]

Can anybody tell me why? Thanks!