diff --git a/src/utils/internal/proxy.ts b/src/utils/internal/proxy.ts
index 0ba530a3..caa71e80 100644
--- a/src/utils/internal/proxy.ts
+++ b/src/utils/internal/proxy.ts
@@ -49,14 +49,17 @@ export function rewriteCookieProperty(
 export function mergeHeaders(
   defaults: HeadersInit,
   ...inputs: (HeadersInit | RequestHeaders | undefined)[]
-) {
+): HeadersInit | Headers {
   const _inputs = inputs.filter(Boolean) as HeadersInit[];
   if (_inputs.length === 0) {
     return defaults;
   }
+
   const merged = new Headers(defaults);
   for (const input of _inputs) {
-    for (const [key, value] of Object.entries(input!)) {
+    const entries =
+      input instanceof Headers ? input.entries() : Object.entries(input!);
+    for (const [key, value] of entries) {
       if (value !== undefined) {
         merged.set(key, value);
       }
diff --git a/test/proxy.test.ts b/test/proxy.test.ts
index 341675e6..e91851cc 100644
--- a/test/proxy.test.ts
+++ b/test/proxy.test.ts
@@ -3,6 +3,7 @@ import { vi, beforeEach } from "vitest";
 import { setCookie } from "../src";
 import { proxy, proxyRequest } from "../src/utils/proxy";
 import { describeMatrix } from "./_setup";
+import { mergeHeaders } from "../src/utils/internal/proxy";
 
 describeMatrix("proxy", (t, { it, expect, describe }) => {
   const spy = vi.spyOn(console, "error");
@@ -187,6 +188,29 @@ describeMatrix("proxy", (t, { it, expect, describe }) => {
         },
         60 * 1000,
       );
+
+      it("can merge input headers into the default", async () => {
+        t.app.all("/test-headers", async (event) => {
+          const headers = new Headers();
+
+          headers.set("Authorization", "Bearer 1234");
+          headers.set("X-Domain", "abcd");
+
+          const mergedHeaders = mergeHeaders(event.request.headers, headers);
+
+          return {
+            headers:
+              mergedHeaders instanceof Headers
+                ? Object.fromEntries(mergedHeaders.entries())
+                : Object.entries(mergedHeaders),
+          };
+        });
+
+        const result = await t.fetch("/test-headers").then((r) => r.json());
+
+        expect(result.headers["authorization"]).toEqual("Bearer 1234");
+        expect(result.headers["x-domain"]).toEqual("abcd");
+      });
     });
 
     describe("multipleCookies", () => {