diff --git a/.changeset/grumpy-bags-stop.md b/.changeset/grumpy-bags-stop.md
new file mode 100644
index 00000000..726acc12
--- /dev/null
+++ b/.changeset/grumpy-bags-stop.md
@@ -0,0 +1,5 @@
+---
+'@hono/medley-router': patch
+---
+
+Add explicit `void` return type
diff --git a/packages/medley-router/src/router.ts b/packages/medley-router/src/router.ts
index f1c10c3e..07976c43 100644
--- a/packages/medley-router/src/router.ts
+++ b/packages/medley-router/src/router.ts
@@ -15,7 +15,7 @@ export class MedleyRouter<T> implements Router<T> {
     this.router = new OriginalRouter()
   }
 
-  add(method: string, path: string, handler: T) {
+  add(method: string, path: string, handler: T): void {
     const store = this.router.register(path)
     store[method] = handler
   }
diff --git a/packages/medley-router/tsconfig.build.json b/packages/medley-router/tsconfig.build.json
index ccc2f65a..a82a1266 100644
--- a/packages/medley-router/tsconfig.build.json
+++ b/packages/medley-router/tsconfig.build.json
@@ -4,7 +4,8 @@
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.build.tsbuildinfo",
-    "emitDeclarationOnly": false
+    "emitDeclarationOnly": false,
+    "isolatedDeclarations": true
   },
   "include": ["src/**/*.ts"],
   "exclude": ["**/*.test.ts"],