diff --git a/CHANGELOG.md b/CHANGELOG.md
index 572c33d1..4d5bff92 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Changelog
 
+- **v4.0.27**
+  - fixed: .toUpperCase issue that prevented serialization of entire router
 - **v4.0.24**
   - fixed: createResponse(undefined) should return undefined (not a Response)
 - **v4.1.0**
diff --git a/package.json b/package.json
index cc32da77..3b385d30 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "itty-router",
-  "version": "4.0.26",
+  "version": "4.0.27",
   "description": "A tiny, zero-dependency router, designed to make beautiful APIs in any environment.",
   "main": "./index.js",
   "module": "./index.mjs",
diff --git a/src/Router.spec.ts b/src/Router.spec.ts
index 64d735be..7f18c70c 100644
--- a/src/Router.spec.ts
+++ b/src/Router.spec.ts
@@ -51,6 +51,12 @@ describe('Router', () => {
     expect(router.routes.length).toBe(3) // accessible off the main router
   })
 
+  it('can serialize router without throwing', () => {
+    const router = Router().get('/', () => 'foo')
+
+    expect(() => console.log(router)).not.toThrow()
+  })
+
   it('router.handle (legacy) is an alias for router.fetch (new)', () => {
     expect(router.fetch).toBe(router.handle)
   })
diff --git a/src/Router.ts b/src/Router.ts
index a84933d1..ac97b7fc 100644
--- a/src/Router.ts
+++ b/src/Router.ts
@@ -88,7 +88,7 @@ export const Router = <
         (route: string, ...handlers: RouteHandler<I>[]) =>
           routes.push(
             [
-              prop.toUpperCase(),
+              prop.toUpperCase?.(),
               RegExp(`^${(path = (base + route)
                 .replace(/\/+(\/|$)/g, '$1'))                       // strip double & trailing splash
                 .replace(/(\/?\.?):(\w+)\+/g, '($1(?<$2>*))')       // greedy params