diff --git a/src/runtime/stores/formatters.ts b/src/runtime/stores/formatters.ts
index ca6eb56..00ee5ec 100644
--- a/src/runtime/stores/formatters.ts
+++ b/src/runtime/stores/formatters.ts
@@ -58,7 +58,7 @@ const formatMessage: MessageFormatter = (id, options = {}) => {
 
   if (!values) return message;
 
-  return getMessageFormatter(message, locale).format(values);
+  return getMessageFormatter(message, locale).format(values) as string;
 };
 
 const formatTime: TimeFormatter = (t, options) =>
diff --git a/src/runtime/types/index.ts b/src/runtime/types/index.ts
index 1cef4c6..e9f8a72 100644
--- a/src/runtime/types/index.ts
+++ b/src/runtime/types/index.ts
@@ -32,7 +32,7 @@ export interface MessageObject {
 export type MessageFormatter = (
   id: string | MessageObject,
   options?: MessageObject,
-) => string | unknown;
+) => string;
 
 export type TimeFormatter = (
   d: Date | number,