Golang Google云端硬盘Oauth2不返回刷新令牌

I am able to go through the oauth procedure to get a token which I'm able to successfully use to to interact with GDrive. The token has an AccessToken but no RefreshToken. How do I get a RefreshToken?

This is in a web service. Here's the code that initiates the oauth authorization procedure:

// Set up a configuration.
oauthconfig := &oauth2.Config{
    ClientID:     XXX,
    ClientSecret: XXX,
    RedirectURL:  "https://MYDOMAIN/gdrivecb",
    Scopes:       []string{"https://www.googleapis.com/auth/userinfo.profile", "https://www.googleapis.com/auth/drive"},
    Endpoint: oauth2.Endpoint{
        AuthURL:  "https://accounts.google.com/o/oauth2/auth",
        TokenURL: "https://accounts.google.com/o/oauth2/token",
    },
}
url := oauthconfig.AuthCodeURL(MYSCOPEDATA, oauth2.AccessTypeOffline)
http.Redirect(w, r, url, http.StatusFound)

Here's the relevant code that's called when /gdrivecb is called (oauthconfig is the same as before, code is the code URL parameter:

token, err = oauthconfig.Exchange(nil, code)

That token contains an AccessToken but no RefreshToken. And it works for an hour (the length of Expiry), but then stops working after that.