Flutter : Null error message for Facebook Audience Network banner ad in iOS using Flutter

Question

I'm integrating Facebook Audience Network into my flutter app to display banner ads. However, when I try to load the ad in ios, I receive a null error message in the response. Here's the error object that I'm getting:

{placement_id: IMG_16_9_APP_INSTALL#IOS_PLACEMENT_ID, invalidated: false, error: }

Code:

FacebookBannerAd(
  placementId: Platform.isAndroid
      ? "IMG_16_9_APP_INSTALL#ANDROID_PLACEMENT_ID"
      : "IMG_16_9_APP_INSTALL#IOS_PLACEMENT_ID",
  bannerSize: BannerSize.MEDIUM_RECTANGLE,
  listener: (result, value) {
    print("$result == $value");
  },
),

Has anyone encountered a similar problem with the Facebook Audience Network banner ads on iOS using flutter? What could be the possible causes for this null error message? Any suggestions or insights would be greatly appreciated.

Thank you!