diff --git a/lib/fetch/request.js b/lib/fetch/request.js
index 37f7c0c969f..c17927ee768 100644
--- a/lib/fetch/request.js
+++ b/lib/fetch/request.js
@@ -419,16 +419,10 @@ class Request {
 
       // 4. If headers is a Headers object, then for each header in its header
       // list, append header’s name/header’s value to this’s headers.
-      if (headers instanceof Headers) {
-        // TODO (fix): Why doesn't this work?
-        // for (const [key, val] of headers[kHeadersList]) {
-        //   this[kHeaders].append(key, val)
-        // }
-
-        this[kState].headersList = new HeadersList([
-          ...this[kState].headersList,
-          ...headers[kHeadersList]
-        ])
+      if (headers.constructor.name === 'Headers') {
+        for (const [key, val] of headers[kHeadersList] || headers) {
+          this[kHeaders].append(key, val)
+        }
       } else {
         // 5. Otherwise, fill this’s headers with headers.
         fillHeaders(this[kState].headersList, headers)
@@ -467,8 +461,7 @@ class Request {
       // not contain `Content-Type`, then append `Content-Type`/Content-Type to
       // this’s headers.
       if (contentType && !this[kHeaders].has('content-type')) {
-        this[kHeaders].set('content-type', contentType)
-        this[kState].headersList.set('content-type', contentType)
+        this[kHeaders].append('content-type', contentType)
       }
     }
 
diff --git a/test/fetch/request.js b/test/fetch/request.js
index e37b395258f..6011c4e28af 100644
--- a/test/fetch/request.js
+++ b/test/fetch/request.js
@@ -4,7 +4,8 @@
 
 const { test } = require('tap')
 const {
-  Request
+  Request,
+  Headers
 } = require('../../')
 const { kState } = require('../../lib/fetch/symbols.js')
 const { URLSearchParams } = require('url')
@@ -294,3 +295,35 @@ test('post aborted signal cloned', t => {
   })
   ac.abort()
 })
+
+test('Passing headers in init', (t) => {
+  // https://github.com/nodejs/undici/issues/1400
+  t.test('Headers instance', (t) => {
+    const req = new Request('http://localhost', {
+      headers: new Headers({ key: 'value' })
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.test('key:value object', (t) => {
+    const req = new Request('http://localhost', {
+      headers: { key: 'value' }
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.test('[key, value][]', (t) => {
+    const req = new Request('http://localhost', {
+      headers: [['key', 'value']]
+    })
+
+    t.equal(req.headers.get('key'), 'value')
+    t.end()
+  })
+
+  t.end()
+})