multipart: NextPart: EOF error for multipart/mixed content type

I was trying the read the mail using go-imap and go-message libraries, but for a particular mail I am getting
`multipart: NextPart: EOF` error on `part, err := reader.NextPart()` where reader is the `mail.CreateReader`.

content type of the email is as following. I got it from gmail client `show original mail`

```
Content-Type: multipart/mixed; boundary="-----SECBOUND"

-------SECBOUND
Content-Type: text/html
Content-Transfer-Encoding: 8bit

html content only

-------SECBOUND--
```