Google Admob causing freezing and "Unable to list voice folder" error

Question

I have several banner ads implemented in my app. I've noticed recently when they initially load, the app briefly freezes and I get this bizarre error:

#FactoryInstall Unable to query results, error: 5 Unable to list voice folder

I have no idea what this could possibly mean - it seems to be something to do with AVSpeechSynthesizer but I am not using that anywhere in my code. I know it's admob because when disable the ads this doesn't happen. Also, if I have a banner ad in a collection view that's initially out of view, and I scroll past it, I see the freezing occurring (as it's loading).

My implementation is pretty straightforward I think. I just create a banner ad and then load it using this function

struct AdMobManager {
    static func loadBannerAd(adView: GADBannerView, id: String) {             
        adView.adUnitID = id
        adView.rootViewController = UIApplication.shared.keyWindow?.rootViewController
        adView.isAutoloadEnabled = true
    }
}

Any ideas?