diff --git a/packages/utils/saferKorean.ts b/packages/utils/saferKorean.ts
index 02cb14b..903fbc1 100644
--- a/packages/utils/saferKorean.ts
+++ b/packages/utils/saferKorean.ts
@@ -205,7 +205,7 @@ export function saferKorean(input: string): string {
 
         // Process number, unit
         .replace(/(\+\d+[\s\-]+)?([\d\-]+)/g, (_, prefix: string, phone: string) => {
-            const all = prefix + phone;
+            const all = (prefix ?? "") + phone;
             if (!phone.includes("-")) return all;
             return PhoneNumberKorean.convert(all);
         })