diff --git a/osu.Game/Online/Rooms/BeatmapAvailability.cs b/osu.Game/Online/Rooms/BeatmapAvailability.cs
index 4ce797e583..2adeb9b959 100644
--- a/osu.Game/Online/Rooms/BeatmapAvailability.cs
+++ b/osu.Game/Online/Rooms/BeatmapAvailability.cs
@@ -22,6 +22,7 @@ namespace osu.Game.Online.Rooms
         /// <summary>
         /// The beatmap's downloading progress, null when not in <see cref="DownloadState.Downloading"/> state.
         /// </summary>
+        [Key(1)]
         public readonly float? DownloadProgress;
 
         [JsonConstructor]