diff --git a/src/types.ts b/src/types.ts
index 0767073c5..c7b027961 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -4902,7 +4902,7 @@ export interface ZodBrandedDef<T extends ZodTypeAny> extends ZodTypeDef {
 
 export const BRAND: unique symbol = Symbol("zod_brand");
 export type BRAND<T extends string | number | symbol> = {
-  [BRAND]: { [k in T]: true };
+  readonly [BRAND]: { readonly [k in T]: true };
 };
 
 export class ZodBranded<