diff --git a/osu.Game.Rulesets.Catch/CatchRuleset.cs b/osu.Game.Rulesets.Catch/CatchRuleset.cs
index d49e9c7c26..4dbe65b3ce 100644
--- a/osu.Game.Rulesets.Catch/CatchRuleset.cs
+++ b/osu.Game.Rulesets.Catch/CatchRuleset.cs
@@ -101,7 +101,7 @@ namespace osu.Game.Rulesets.Catch
 
         public override DifficultyCalculator CreateDifficultyCalculator(Beatmap beatmap, Mod[] mods = null) => new CatchDifficultyCalculator(beatmap);
 
-        public override int LegacyID => 2;
+        public override int? LegacyID => 2;
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new CatchReplayFrame();
 
diff --git a/osu.Game.Rulesets.Mania/ManiaRuleset.cs b/osu.Game.Rulesets.Mania/ManiaRuleset.cs
index 3ad498e6ea..16b6888f2b 100644
--- a/osu.Game.Rulesets.Mania/ManiaRuleset.cs
+++ b/osu.Game.Rulesets.Mania/ManiaRuleset.cs
@@ -114,7 +114,7 @@ namespace osu.Game.Rulesets.Mania
 
         public override DifficultyCalculator CreateDifficultyCalculator(Beatmap beatmap, Mod[] mods = null) => new ManiaDifficultyCalculator(beatmap);
 
-        public override int LegacyID => 3;
+        public override int? LegacyID => 3;
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new ManiaReplayFrame();
 
diff --git a/osu.Game.Rulesets.Osu/OsuRuleset.cs b/osu.Game.Rulesets.Osu/OsuRuleset.cs
index 3f0aea5cb2..d407835a96 100644
--- a/osu.Game.Rulesets.Osu/OsuRuleset.cs
+++ b/osu.Game.Rulesets.Osu/OsuRuleset.cs
@@ -145,7 +145,7 @@ namespace osu.Game.Rulesets.Osu
 
         public override SettingsSubsection CreateSettings() => new OsuSettings();
 
-        public override int LegacyID => 0;
+        public override int? LegacyID => 0;
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new OsuReplayFrame();
 
diff --git a/osu.Game.Rulesets.Taiko/TaikoRuleset.cs b/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
index 713506e831..0a9719f27b 100644
--- a/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
+++ b/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
@@ -103,7 +103,7 @@ namespace osu.Game.Rulesets.Taiko
 
         public override DifficultyCalculator CreateDifficultyCalculator(Beatmap beatmap, Mod[] mods = null) => new TaikoDifficultyCalculator(beatmap);
 
-        public override int LegacyID => 1;
+        public override int? LegacyID => 1;
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new TaikoReplayFrame();
 
diff --git a/osu.Game/Rulesets/Ruleset.cs b/osu.Game/Rulesets/Ruleset.cs
index f9b64995f9..cba849a491 100644
--- a/osu.Game/Rulesets/Ruleset.cs
+++ b/osu.Game/Rulesets/Ruleset.cs
@@ -64,7 +64,7 @@ namespace osu.Game.Rulesets
         /// <summary>
         /// Do not override this unless you are a legacy mode.
         /// </summary>
-        public virtual int LegacyID => -1;
+        public virtual int? LegacyID => null;
 
         /// <summary>
         /// A unique short name to reference this ruleset in online requests.