Advertisement.IsReady() returns false, but GetPlacementState returns "Ready"????

I don't understand the relationship between IsReady and GetPlacementState.

This works fine in the Unity Editor (that is, IsReady() returns true and shows me the test ad) but on Android I'm getting this result:

• IsReady() returns false
• immediately checking GetPlacementState()... returns "Ready".

Code (CSharp):

1. if (Advertisement.IsReady()) {
2.     Advertisement.Show();
3. } else {
4.     // actually, this outputs to an onscreen field, but it's equivalent to Debug.Log
5.     Debug.Log("ad fail ["+placementId+"]: "+Advertisement.GetPlacementState(placementId));
6. }

The result is
ad fail [my_interstitial]: Ready

Is there a more/better way to find out WHY IsReady() is returning false?

 

OK, I answered my own question. I'm missing the placementId parameter to IsReady() and to Show(). It should be

Code (CSharp):

1. if (Advertisement.IsReady(placementId)) {
2.     Advertisement.Show(placementId);

But I claim I was misled by Unity's documentation that doesn't show these placementId parameters in the interstitial example at https://unityads.unity3d.com/help/unity/integration-guide-unity#basic-ads-implementation