Why does AdMob return NO FILL

I'm using an AdMob view in an android app, but I'm unable to get any ad into the app.

For reference, I've added the view to a ListView as explained by Dan Dyer here

EDIT:I'm using GoogleAdMobAdsSdk-4.1.1. The release notes of Google AdMob Ads SDK for version 4.1.0 says:

"...- Added support for AdRequest.addTestDevice() and AdRequest.setTestDevices(). Note that AdRequest.setTesting() is now deprecated...."

This is how the ad is inserted to my ListView:

public View getView(int position, View convertView, ViewGroup parent) {  // Some other code  // Reusing convertView etc.  AdView adView =      new AdView((Activity) getContext(), AdSize.BANNER, "/xxxxxx/ca-pub-xxxxxxx/my_ad_unit");  for (int i = 0; i < adView.getChildCount(); i++) {    adView.getChildAt(i).setFocusable(false);  }  adView.setFocusable(false);  float density = getContext().getResources().getDisplayMetrics().density;  int height = Math.round(50 * density);  AbsListView.LayoutParams params = new AbsListView.LayoutParams(    AbsListView.LayoutParams.FILL_PARENT, height);  adView.setLayoutParams(params);  AdRequest request = new AdRequest();  request.addTestDevice("xxxxxxxxxxxxxxxxx");  adView.loadAd(request);  // other stuff  // returning convertView}

I've also added an AdListener to the adview, and on every loadAd, the onFailedToReceiveAd callback method is called:

public void onFailedToReceiveAd(Ad arg0, ErrorCode arg1) {  Log.d(TAG, "AdMob in list failed to receive ad: "+ arg1.name());}

In logcat I get this message:

08-17 15:22:18.065: AdMob in list failed to receive ad: NO_FILL

Can anyone tell me what this errorcode means?