diff --git a/deno_dist/hono-base.ts b/deno_dist/hono-base.ts
index 81f3aa51..faf9ce19 100644
--- a/deno_dist/hono-base.ts
+++ b/deno_dist/hono-base.ts
@@ -174,12 +174,12 @@ class Hono<
     return subApp
   }
 
-  onError(handler: ErrorHandler<E>) {
+  onError = (handler: ErrorHandler<E>) => {
     this.errorHandler = handler
     return this
   }
 
-  notFound(handler: NotFoundHandler<E>) {
+  notFound = (handler: NotFoundHandler<E>) => {
     this.notFoundHandler = handler
     return this
   }
diff --git a/src/hono-base.ts b/src/hono-base.ts
index b8c0e7a5..c9c30e64 100644
--- a/src/hono-base.ts
+++ b/src/hono-base.ts
@@ -174,12 +174,12 @@ class Hono<
     return subApp
   }
 
-  onError(handler: ErrorHandler<E>) {
+  onError = (handler: ErrorHandler<E>) => {
     this.errorHandler = handler
     return this
   }
 
-  notFound(handler: NotFoundHandler<E>) {
+  notFound = (handler: NotFoundHandler<E>) => {
     this.notFoundHandler = handler
     return this
   }