diff --git a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan.cs b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan.cs
index 001c9a130..15bf8501d 100644
--- a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan.cs
+++ b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan.cs
@@ -9,6 +9,15 @@ public abstract class Zukan
         protected int PokeDex { get; set; }
         protected int PokeDexLanguageFlags { get; set; }
 
+        protected Zukan(SaveFile sav, int dex, int langflag)
+        {
+            SAV = sav;
+            PokeDex = dex;
+            PokeDexLanguageFlags = langflag;
+            if (langflag > dex)
+                throw new ArgumentException(nameof(langflag));
+        }
+
         protected abstract int OFS_SEEN { get; }
         protected abstract int OFS_CAUGHT { get; }
         protected abstract int BitSeenSize { get; }
diff --git a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan5.cs b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan5.cs
index e6d2eb9bc..1220ce0d1 100644
--- a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan5.cs
+++ b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan5.cs
@@ -10,11 +10,8 @@ public class Zukan5 : Zukan
         protected override int DexLangFlagByteCount => 7;
         protected override int DexLangIDCount => 7;
 
-        public Zukan5(SaveFile sav, int dex, int langflag)
+        public Zukan5(SaveFile sav, int dex, int langflag) : base(sav, dex, langflag - dex)
         {
-            SAV = sav;
-            PokeDex = dex;
-            PokeDexLanguageFlags = langflag;
             var wrap = SAV.BW ? DexFormUtil.GetDexFormIndexBW : (Func<int, int, int>)DexFormUtil.GetDexFormIndexB2W2;
             DexFormIndexFetcher = (spec, form, _) => wrap(spec, form);
         }
diff --git a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan6.cs b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan6.cs
index 0fb30d91e..dad27d265 100644
--- a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan6.cs
+++ b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan6.cs
@@ -10,11 +10,8 @@ public class Zukan6 : Zukan
         protected override int DexLangFlagByteCount => 7;
         protected override int DexLangIDCount => 7;
 
-        public Zukan6(SaveFile sav, int dex, int langflag)
+        public Zukan6(SaveFile sav, int dex, int langflag) : base(sav, dex, langflag - dex)
         {
-            SAV = sav;
-            PokeDex = dex;
-            PokeDexLanguageFlags = langflag;
             var wrap = SAV.ORAS ? DexFormUtil.GetDexFormIndexORAS : (Func<int,int,int>)DexFormUtil.GetDexFormIndexXY;
             DexFormIndexFetcher = (spec, form, _) => wrap(spec, form);
         }
diff --git a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7.cs b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7.cs
index 01eec4cc3..b5ad46d03 100644
--- a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7.cs
+++ b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7.cs
@@ -21,15 +21,8 @@ public class Zukan7 : Zukan
 
         private IList<int> FormBaseSpecies;
 
-        protected Zukan7()
+        public Zukan7(SaveFile sav, int dex, int langflag) : base(sav, dex, langflag - dex)
         {
-        }
-
-        public Zukan7(SaveFile sav, int dex, int langflag)
-        {
-            SAV = sav;
-            PokeDex = dex;
-            PokeDexLanguageFlags = langflag;
             DexFormIndexFetcher = SAV.USUM ? (Func<int, int, int, int>) DexFormUtil.GetDexFormIndexSM : DexFormUtil.GetDexFormIndexSM;
             LoadDexList();
             Debug.Assert(!SAV.Exportable || BitConverter.ToUInt32(SAV.Data, PokeDex) == MAGIC);
diff --git a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7b.cs b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7b.cs
index 515b280a7..57b595e21 100644
--- a/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7b.cs
+++ b/PKHeX.Core/Saves/Substructures/PokeDex/Zukan7b.cs
@@ -19,11 +19,8 @@ public class Zukan7b : Zukan7
         protected override int DexLangFlagByteCount => 920; // 0x398 = 817*9, top off the savedata block.
         protected override int DexLangIDCount => 9; // CHT, skipping langID 6 (unused)
 
-        public Zukan7b(SaveFile sav, int dex, int langflag)
+        public Zukan7b(SaveFile sav, int dex, int langflag) : base(sav, dex, langflag)
         {
-            SAV = sav;
-            PokeDex = dex;
-            PokeDexLanguageFlags = langflag;
             DexFormIndexFetcher = DexFormUtil.GetDexFormIndexGG;
             LoadDexList();
         }