diff --git a/app/src/play/java/eu/faircode/email/ActivityBilling.java b/app/src/play/java/eu/faircode/email/ActivityBilling.java
index 214eb9ae78..31a0260236 100644
--- a/app/src/play/java/eu/faircode/email/ActivityBilling.java
+++ b/app/src/play/java/eu/faircode/email/ActivityBilling.java
@@ -169,6 +169,8 @@ public class ActivityBilling extends ActivityBase implements PurchasesUpdatedLis
 
     private static String getChallenge(Context context) throws NoSuchAlgorithmException {
         String android_id = Settings.Secure.getString(context.getContentResolver(), Settings.Secure.ANDROID_ID);
+        if (android_id == null)
+            android_id = Long.toHexString(System.currentTimeMillis() % (24 * 3600 * 1000L));
         return Helper.sha256(android_id);
     }