diff --git a/PKHeX.Core/Legality/Encounters/Generator/EncounterSlotGenerator.cs b/PKHeX.Core/Legality/Encounters/Generator/EncounterSlotGenerator.cs
index 5e1bd2eee..aeaf86f63 100644
--- a/PKHeX.Core/Legality/Encounters/Generator/EncounterSlotGenerator.cs
+++ b/PKHeX.Core/Legality/Encounters/Generator/EncounterSlotGenerator.cs
@@ -93,6 +93,8 @@ public static IEnumerable<EncounterSlot> GetValidFriendSafari(PKM pkm)
         {
             if (!pkm.XY || pkm.Met_Location != 148 || pkm.Met_Level != 30) // Friend Safari
                 return Enumerable.Empty<EncounterSlot>();
+            if (pkm.Species == 423 && pkm.AltForm != 0)
+                return Enumerable.Empty<EncounterSlot>();
             var vs = EvolutionChain.GetValidPreEvolutions(pkm).Where(d => d.Level >= 30);
             return vs.SelectMany(z => Encounters6.FriendSafari[z.Species]);
         }
diff --git a/Tests/PKHeX.Tests/Legality/Illegal/AltForm/423-01 - Gastrodon - 6CE785FF60E2.pk7 b/Tests/PKHeX.Tests/Legality/Illegal/AltForm/423-01 - Gastrodon - 6CE785FF60E2.pk7
new file mode 100644
index 000000000..ec0b1b322
Binary files /dev/null and b/Tests/PKHeX.Tests/Legality/Illegal/AltForm/423-01 - Gastrodon - 6CE785FF60E2.pk7 differ