diff --git a/osu.Game/Beatmaps/Formats/Decoder.cs b/osu.Game/Beatmaps/Formats/Decoder.cs
index 8eb238a184..c1537d7240 100644
--- a/osu.Game/Beatmaps/Formats/Decoder.cs
+++ b/osu.Game/Beatmaps/Formats/Decoder.cs
@@ -5,6 +5,7 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
+using JetBrains.Annotations;
 using osu.Game.IO;
 using osu.Game.Rulesets;
 
@@ -42,9 +43,9 @@ static Decoder()
         /// Register dependencies for use with static decoder classes.
         /// </summary>
         /// <param name="rulesets">A store containing all available rulesets (used by <see cref="LegacyBeatmapDecoder"/>).</param>
-        public static void RegisterDependencies(RulesetStore rulesets)
+        public static void RegisterDependencies([NotNull] RulesetStore rulesets)
         {
-            LegacyBeatmapDecoder.RulesetStore = rulesets;
+            LegacyBeatmapDecoder.RulesetStore = rulesets ?? throw new ArgumentNullException(nameof(rulesets));
         }
 
         /// <summary>