diff --git a/osu.Game/Beatmaps/Formats/OsuLegacyDecoder.cs b/osu.Game/Beatmaps/Formats/OsuLegacyDecoder.cs
index f231342234..d240b7669c 100644
--- a/osu.Game/Beatmaps/Formats/OsuLegacyDecoder.cs
+++ b/osu.Game/Beatmaps/Formats/OsuLegacyDecoder.cs
@@ -209,6 +209,11 @@ namespace osu.Game.Beatmaps.Formats
 
         private void handleDifficulty(Beatmap beatmap, string line)
         {
+            if (beatmap == null)
+                throw new ArgumentNullException(nameof(beatmap));
+            if (line == null)
+                throw new ArgumentNullException(nameof(line));
+
             var pair = splitKeyVal(line, ':');
 
             var difficulty = beatmap.BeatmapInfo.BaseDifficulty;