diff --git a/packages/core/src/i18n/i18nTypes.ts b/packages/core/src/i18n/i18nTypes.ts
index e13d904e4..3f318796b 100644
--- a/packages/core/src/i18n/i18nTypes.ts
+++ b/packages/core/src/i18n/i18nTypes.ts
@@ -4,6 +4,7 @@ import { JsonSchema, UISchemaElement } from '../models';
 export type Translator = {
     (id: string, defaultMessage: string, values?: any): string;
     (id: string, defaultMessage: undefined, values?: any): string | undefined;
+    (id: string, defaultMessage?: string, values?: any): string | undefined;
 }
 
 export type ErrorTranslator = (error: ErrorObject, translate: Translator, uischema?: UISchemaElement) => string;