diff --git a/src/__test__/fetch-util.spec.js b/src/__test__/fetch-util.spec.js
new file mode 100644
index 0000000..509f72f
--- /dev/null
+++ b/src/__test__/fetch-util.spec.js
@@ -0,0 +1,28 @@
+// @flow
+/* eslint-env jest */
+
+import { copyHeaders } from '../fetch-util'
+import { polyfillWindow, polyunfillWindow } from './spec-helpers'
+
+describe('copyHeaders', () => {
+  beforeEach(() => polyfillWindow())
+  afterEach(() => polyunfillWindow())
+
+  it('returns an object', () => expect(copyHeaders({})).toEqual({}))
+  it('returns a copy of whatever is passed in as first param', () =>
+    expect(copyHeaders({ foo: 42 })).toEqual({ foo: 42 }))
+  it('handles option.headers as object', () =>
+    expect(copyHeaders({ foo: 42 }, { headers: { bar: 1337 } })).toEqual({
+      foo: 42,
+      bar: 1337
+    }))
+  xit('handles options.headers as Headers', () => {
+    // TODO: Need to have Headers working
+    const headers = new window.Headers()
+    headers.append('bar', '1337')
+    expect(copyHeaders({ foo: '42' }, { headers })).toEqual({
+      foo: '42',
+      bar: '1337'
+    })
+  })
+})
diff --git a/src/__test__/spec-helpers.js b/src/__test__/spec-helpers.js
index e1c4527..b1a17d5 100644
--- a/src/__test__/spec-helpers.js
+++ b/src/__test__/spec-helpers.js
@@ -31,6 +31,23 @@ export const polyfillWindow = () => {
     value: window.origin
   })
   MessageEvent.prototype.origin = window.location.origin
+
+  // in case Headers is not available
+  // Object.defineProperty(window, 'Headers', {
+  //   value: class Headers {
+  //     constructor() {
+  //       this.map = {}
+  //     }
+  //
+  //     append(key, value) {
+  //       this.map[key] = value
+  //     }
+  //
+  //     forEach(callback) {
+  //       Object.keys(this.map).forEach(key => callback(this.map[key], key))
+  //     }
+  //   }
+  // })
 }
 
 export const polyunfillWindow = () => {
diff --git a/src/fetch-util.js b/src/fetch-util.js
new file mode 100644
index 0000000..43983e8
--- /dev/null
+++ b/src/fetch-util.js
@@ -0,0 +1,17 @@
+export function copyHeaders(newHeaders, oldOptions = {}) {
+  return {
+    ...normalizeHeaders(oldOptions.headers),
+    ...normalizeHeaders(newHeaders)
+  }
+}
+
+function normalizeHeaders(headers = {}) {
+  if (!headers.forEach) {
+    return headers
+  }
+  const newHeaders = {}
+  headers.forEach((value, key) => {
+    newHeaders[key] = value
+  })
+  return newHeaders
+}
diff --git a/src/webid-oidc.js b/src/webid-oidc.js
index a23d25c..75b3a97 100644
--- a/src/webid-oidc.js
+++ b/src/webid-oidc.js
@@ -9,6 +9,7 @@ import { currentUrl, navigateTo, toUrlString } from './url-util'
 import type { webIdOidcSession } from './session'
 import type { AsyncStorage } from './storage'
 import { defaultStorage, getData, updateStorage } from './storage'
+import { copyHeaders } from './fetch-util'
 
 export async function login(
   idp: string,
@@ -191,10 +192,12 @@ export async function fetchWithCredentials(
   const authenticatedOptions = {
     ...options,
     credentials: 'include',
-    headers: {
-      ...(options && options.headers ? options.headers : {}),
-      authorization: `Bearer ${popToken}`
-    }
+    headers: copyHeaders(
+      {
+        authorization: `Bearer ${popToken}`
+      },
+      options
+    )
   }
   return fetch(input, authenticatedOptions)
 }