Google App Engine Go - Reddit API returns error 429

I am trying to create a Reddit API in Golang in Google App Engine. My code:

package RedditAPI

import(
	"appengine"
	"encoding/json"
	"io/ioutil"
	"net/http"
    "appengine/urlfetch"
    "time"
	"net/url"
)

func GetTopSubmissions(c appengine.Context){
	one, two:=Call(c, "http://www.reddit.com/r/Bitcoin/top.json", "POST", nil);
	c.Infof("%v, %v", one, two);
}


func Call(c appengine.Context, address string, requestType string, values url.Values)(map[string]interface{}, error){
    req, err:=http.NewRequest("GET", address, nil)
    if err!=nil{
        c.Infof("Request: %v", err)
    	return nil, err
    }
    
    req.Header.Add("User-Agent", "This is a very creative name for a Reddit bot v1.0 by /u/username")
    c.Infof("%v", req.Header.Get("User-Agent"))
    c.Infof("%v", req)
    c.Infof("%v", req.UserAgent())
    
    duration, err:= time.ParseDuration("60s")
    tr := &urlfetch.Transport{Context: c, Deadline: duration}
    
    resp, err:=tr.RoundTrip(req)
    if err != nil {
        c.Infof("Post: %v", err)
    	return nil, err
    }
    defer resp.Body.Close()
    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        c.Infof("ReadAll: %v", err)
    	return nil, err
    }
    result := make(map[string]interface{})
    err = json.Unmarshal(body, &result)
    if err != nil {
        c.Infof("Unmarshal: %v", err)
        c.Infof("%s", body)
    	return nil, err
    }
    return result, nil
}

Returns

2013/05/23 03:00:34 Unsolicited response received on idle HTTP channel starting with "H"; err=<nil>
2013/05/23 03:01:42 INFO: &{GET http://www.reddit.com/r/Bitcoin/top.json HTTP/1.1 1 1 map[User-Agent:[This is a very creative name for a Reddit bot v1.0 by /u/username]] <nil> 0 [] false www.reddit.com map[] <nil> map[]   <nil>}
2013/05/23 03:01:42 INFO: This is a very creative name for a Reddit bot v1.0 by /u/username
2013/05/23 03:01:42 INFO: map[error:429], <nil>
INFO     2013-05-23 03:01:42,720 server.py:584] default: "GET / HTTP/1.1" 200 81

What can be the cause of this error?

HTTP error 429 is 'too many requests': https://www.rfc-editor.org/rfc/rfc6585#section-4

At https://github.com/reddit/reddit/wiki/API, Reddit says "Make no more than thirty requests per minute. This allows some burstiness to your requests, but keep it sane. On average, we should see no more than one request every two seconds from you." They also say, "Don't hit the same page more than once per 30 seconds."

If you're requesting the same resource on every user hit to your app, you could use GAE's memcache support. If you're requesting a bunch of resources, the Reddit wiki page mentions "[r]equests for multiple resources at a time", so there may be some facility for batching requests (not sure, not versed in the API). If you're doing a cronjob, you could run it less frequently.

In any case, the remaining mystery is more of a Reddit API question than a Go question. The repo matthewbauer commented with (https://github.com/jzelinskie/reddit) or the Reddit API docs may have more.