diff --git a/lib/http/lookup-url.ts b/lib/http/lookup-url.ts
index deb038e..015db2a 100644
--- a/lib/http/lookup-url.ts
+++ b/lib/http/lookup-url.ts
@@ -6,7 +6,7 @@ import { CONFIG, isValidASN } from "$common/mod.ts";
  * @returns the URL to lookup the ASN if the ASN lookup is enabled
  * @throws {Error} when the ASN lookup is disabled or the ASN is invalid
  */
-export function getLookupURL(asn: string) {
+export function getLookupURL(asn: string): string {
   const baseUrl = CONFIG.ASN_LOOKUP_URL;
 
   if (!baseUrl) {