diff --git a/Classes/Finishers/BrevoFinisher.php b/Classes/Finishers/BrevoFinisher.php
index d5a8629..a74776a 100644
--- a/Classes/Finishers/BrevoFinisher.php
+++ b/Classes/Finishers/BrevoFinisher.php
@@ -139,7 +139,9 @@ protected function getEnrichedListIds(): array
                 }
             }
         }
-        return array_unique($lists);
+
+        // Make sure a list is returned - see: https://github.com/studiomitte/brevo/issues/19
+        return array_values(array_unique($lists));
     }
 
     /**