diff --git a/osu.Game/Beatmaps/BeatmapManager.cs b/osu.Game/Beatmaps/BeatmapManager.cs
index 784d440584..a23fab5393 100644
--- a/osu.Game/Beatmaps/BeatmapManager.cs
+++ b/osu.Game/Beatmaps/BeatmapManager.cs
@@ -308,7 +308,10 @@ namespace osu.Game.Beatmaps
             var hash = hashable.ComputeSHA2Hash();
 
             // check if this beatmap has already been imported and exit early if so.
-            var beatmapSet = beatmaps.QueryAndPopulate<BeatmapSetInfo>(b => b.Hash == hash).FirstOrDefault();
+            BeatmapSetInfo beatmapSet;
+            lock (beatmaps)
+                beatmapSet = beatmaps.QueryAndPopulate<BeatmapSetInfo>(b => b.Hash == hash).FirstOrDefault();
+
             if (beatmapSet != null)
             {
                 Undelete(beatmapSet);