diff --git a/src/predicate/isJSONable.ts b/src/predicate/isJSONable.ts
index c13cdba..84dcf2f 100644
--- a/src/predicate/isJSONable.ts
+++ b/src/predicate/isJSONable.ts
@@ -1,6 +1,6 @@
 import type { JsonValue } from '../types/common.js';
 
-export const isJSONable = (input: unknown): input is { toJSON: (key: string) => JsonValue } => {
+export const isJSONable = (input: unknown): input is { toJSON: (key?: string) => JsonValue } => {
   return (
     input !== null &&
     typeof input === 'object' &&