diff --git a/src/data/ability.ts b/src/data/ability.ts
index 08dc1ed27a4..736f5862530 100644
--- a/src/data/ability.ts
+++ b/src/data/ability.ts
@@ -2463,12 +2463,15 @@ export class PostSummonCopyAllyStatsAbAttr extends PostSummonAbAttr {
   }
 }
 
+/**
+ * Used by Imposter
+ */
 export class PostSummonTransformAbAttr extends PostSummonAbAttr {
   constructor() {
     super(true);
   }
 
-  async applyPostSummon(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]): Promise<boolean> {
+  async applyPostSummon(pokemon: Pokemon, _passive: boolean, simulated: boolean, _args: any[]): Promise<boolean> {
     const targets = pokemon.getOpponents();
     if (simulated || !targets.length) {
       return simulated;
@@ -2477,17 +2480,31 @@ export class PostSummonTransformAbAttr extends PostSummonAbAttr {
 
     let target: Pokemon;
     if (targets.length > 1) {
-      pokemon.scene.executeWithSeedOffset(() => target = Utils.randSeedItem(targets), pokemon.scene.currentBattle.waveIndex);
+      pokemon.scene.executeWithSeedOffset(() => {
+        // in a double battle, if one of the opposing pokemon is fused the other one will be chosen
+        // if both are fused, then Imposter will fail below
+        if (targets[0].fusionSpecies) {
+          target = targets[1];
+          return;
+        } else if (targets[1].fusionSpecies) {
+          target = targets[0];
+          return;
+        }
+        target = Utils.randSeedItem(targets);
+      }, pokemon.scene.currentBattle.waveIndex);
     } else {
       target = targets[0];
     }
-
     target = target!;
+
+    // transforming from or into fusion pokemon causes various problems (including crashes and save corruption)
+    if (target.fusionSpecies || pokemon.fusionSpecies) {
+      return false;
+    }
+
     pokemon.summonData.speciesForm = target.getSpeciesForm();
-    pokemon.summonData.fusionSpeciesForm = target.getFusionSpeciesForm();
     pokemon.summonData.ability = target.getAbility().id;
     pokemon.summonData.gender = target.getGender();
-    pokemon.summonData.fusionGender = target.getFusionGender();
 
     // Copy all stats (except HP)
     for (const s of EFFECTIVE_STATS) {
diff --git a/src/data/move.ts b/src/data/move.ts
index ed2b176f54c..37afe651861 100644
--- a/src/data/move.ts
+++ b/src/data/move.ts
@@ -6995,6 +6995,9 @@ export class SuppressAbilitiesIfActedAttr extends MoveEffectAttr {
   }
 }
 
+/**
+ * Used by Transform
+ */
 export class TransformAttr extends MoveEffectAttr {
   async apply(user: Pokemon, target: Pokemon, move: Move, args: any[]): Promise<boolean> {
     if (!super.apply(user, target, move, args)) {
@@ -7003,10 +7006,8 @@ export class TransformAttr extends MoveEffectAttr {
 
     const promises: Promise<void>[] = [];
     user.summonData.speciesForm = target.getSpeciesForm();
-    user.summonData.fusionSpeciesForm = target.getFusionSpeciesForm();
     user.summonData.ability = target.getAbility().id;
     user.summonData.gender = target.getGender();
-    user.summonData.fusionGender = target.getFusionGender();
 
     // Power Trick's effect will not preserved after using Transform
     user.removeTag(BattlerTagType.POWER_TRICK);
@@ -8077,7 +8078,8 @@ export function initMoves() {
       .ignoresVirtual(),
     new StatusMove(Moves.TRANSFORM, Type.NORMAL, -1, 10, -1, 0, 1)
       .attr(TransformAttr)
-      .condition((user, target, move) => !target.getTag(BattlerTagType.SUBSTITUTE))
+      // transforming from or into fusion pokemon causes various problems (such as crashes)
+      .condition((user, target, move) => !target.getTag(BattlerTagType.SUBSTITUTE) && !user.fusionSpecies && !target.fusionSpecies)
       .ignoresProtect(),
     new AttackMove(Moves.BUBBLE, Type.WATER, MoveCategory.SPECIAL, 40, 100, 30, 10, 0, 1)
       .attr(StatStageChangeAttr, [ Stat.SPD ], -1)
diff --git a/src/data/pokemon-species.ts b/src/data/pokemon-species.ts
index e7fe902956c..ff53fdb9392 100644
--- a/src/data/pokemon-species.ts
+++ b/src/data/pokemon-species.ts
@@ -888,17 +888,24 @@ export default class PokemonSpecies extends PokemonSpeciesForm implements Locali
   getCompatibleFusionSpeciesFilter(): PokemonSpeciesFilter {
     const hasEvolution = pokemonEvolutions.hasOwnProperty(this.speciesId);
     const hasPrevolution = pokemonPrevolutions.hasOwnProperty(this.speciesId);
-    const pseudoLegendary = this.subLegendary;
+    const subLegendary = this.subLegendary;
     const legendary = this.legendary;
     const mythical = this.mythical;
     return species => {
-      return (pseudoLegendary || legendary || mythical ||
-        (pokemonEvolutions.hasOwnProperty(species.speciesId) === hasEvolution
-        && pokemonPrevolutions.hasOwnProperty(species.speciesId) === hasPrevolution))
-        && species.subLegendary === pseudoLegendary
+      return (
+        subLegendary
+        || legendary
+        || mythical
+        || (
+          pokemonEvolutions.hasOwnProperty(species.speciesId) === hasEvolution
+          && pokemonPrevolutions.hasOwnProperty(species.speciesId) === hasPrevolution
+        )
+      )
+        && species.subLegendary === subLegendary
         && species.legendary === legendary
         && species.mythical === mythical
-        && (this.isTrainerForbidden() || !species.isTrainerForbidden());
+        && (this.isTrainerForbidden() || !species.isTrainerForbidden())
+        && species.speciesId !== Species.DITTO;
     };
   }
 
diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index daa73164577..d413e618381 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -2030,15 +2030,17 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
     const hasHiddenAbility = !Utils.randSeedInt(hiddenAbilityChance.value);
     const randAbilityIndex = Utils.randSeedInt(2);
 
-    const filter = !forStarter ? this.species.getCompatibleFusionSpeciesFilter()
-      : species => {
+    const filter = !forStarter ?
+      this.species.getCompatibleFusionSpeciesFilter()
+      : (species: PokemonSpecies) => {
         return pokemonEvolutions.hasOwnProperty(species.speciesId)
-      && !pokemonPrevolutions.hasOwnProperty(species.speciesId)
-      && !species.pseudoLegendary
-      && !species.legendary
-      && !species.mythical
-      && !species.isTrainerForbidden()
-      && species.speciesId !== this.species.speciesId;
+          && !pokemonPrevolutions.hasOwnProperty(species.speciesId)
+          && !species.subLegendary
+          && !species.legendary
+          && !species.mythical
+          && !species.isTrainerForbidden()
+          && species.speciesId !== this.species.speciesId
+          && species.speciesId !== Species.DITTO;
       };
 
     let fusionOverride: PokemonSpecies | undefined = undefined;