diff --git a/config/formats.ts b/config/formats.ts
index 67c592624b..831f50520b 100644
--- a/config/formats.ts
+++ b/config/formats.ts
@@ -1213,7 +1213,12 @@ export const Formats: import('../sim/dex-formats').FormatList = [
 			if (species.requiredItems) requiredItems = species.requiredItems;
 			if (species.requiredMove) requiredMove = species.requiredMove;
 			if (species.requiredAbility) requiredAbility = species.requiredAbility;
-			if (species.battleOnly) species = dex.species.get(species.battleOnly as string);
+			if (species.battleOnly) {
+				if (typeof species.battleOnly !== 'string') {
+					return this.validateForme(set) || [`${species.name} does not start the battle in that forme.`];
+				}
+				species = dex.species.get(species.battleOnly);
+			}
 
 			const effectFunctions = [dex.abilities, dex.items, dex.moves];
 			if (
diff --git a/data/rulesets.ts b/data/rulesets.ts
index 8cae505815..8936933496 100644
--- a/data/rulesets.ts
+++ b/data/rulesets.ts
@@ -2786,8 +2786,13 @@ export const Rulesets: import('../sim/dex-formats').FormatDataTable = {
 				if (species.requiredMove && !set.moves.map(this.toID).includes(this.toID(species.requiredMove))) {
 					return [`${set.name ? `${set.name} (${species.name})` : species.name} is required to have ${species.requiredMove}.`];
 				}
-				set.species = (species.id === 'xerneas' ? 'Xerneas-Neutral' :
-					species.id === 'zygardecomplete' ? 'Zygarde' : species.battleOnly) as string;
+				set.species = (
+					species.id === 'xerneas' ? 'Xerneas-Neutral' :
+					typeof species.battleOnly === 'string' ? species.battleOnly :
+					species.battleOnly ? species.battleOnly[0] :
+					// should never happen?
+					set.species
+				);
 				species = this.dex.species.get(set.species);
 			}
 			for (const moveid of set.moves) {