diff --git a/src/field/pokemon.ts b/src/field/pokemon.ts
index d6f73e1b5bc..a0ad4e8a52f 100644
--- a/src/field/pokemon.ts
+++ b/src/field/pokemon.ts
@@ -1707,7 +1707,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
     if (pokemonEvolutions.hasOwnProperty(this.species.speciesId)) {
       const evolutions = pokemonEvolutions[this.species.speciesId];
       for (const e of evolutions) {
-        if (!e.item && this.level >= e.level && (!e.preFormKey || this.getFormKey() === e.preFormKey)) {
+        if (!e.item && this.level >= e.level && (isNullOrUndefined(e.preFormKey) || this.getFormKey() === e.preFormKey)) {
           if (e.condition === null || (e.condition as SpeciesEvolutionCondition).predicate(this)) {
             return e;
           }
@@ -1718,7 +1718,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
     if (this.isFusion() && this.fusionSpecies && pokemonEvolutions.hasOwnProperty(this.fusionSpecies.speciesId)) {
       const fusionEvolutions = pokemonEvolutions[this.fusionSpecies.speciesId].map(e => new FusionSpeciesFormEvolution(this.species.speciesId, e));
       for (const fe of fusionEvolutions) {
-        if (!fe.item && this.level >= fe.level && (!fe.preFormKey || this.getFusionFormKey() === fe.preFormKey)) {
+        if (!fe.item && this.level >= fe.level && (isNullOrUndefined(fe.preFormKey) || this.getFusionFormKey() === fe.preFormKey)) {
           if (fe.condition === null || (fe.condition as SpeciesEvolutionCondition).predicate(this)) {
             return fe;
           }