diff --git a/__tests__/response/body.test.js b/__tests__/response/body.test.js
index c63cb4d75..8f684a328 100644
--- a/__tests__/response/body.test.js
+++ b/__tests__/response/body.test.js
@@ -288,6 +288,14 @@ describe('res.body=', () => {
       res.body = new ReadableStream()
       assert.strictEqual('application/octet-stream', res.header['content-type'])
     })
+
+    it('should remove stale Content-Length when replacing a body', () => {
+      const res = response()
+      res.body = 'hello'
+      assert.strictEqual(5, res.length)
+      res.body = new ReadableStream()
+      assert.strictEqual(undefined, res.header['content-length'])
+    })
   })
 
   describe('when a Blob is given', () => {
@@ -324,5 +332,13 @@ describe('res.body=', () => {
       assert.strictEqual(301, res.status)
       assert.strictEqual('https://www.example.com/', res.header.location)
     })
+
+    it('should remove stale Content-Length when replacing a body', () => {
+      const res = response()
+      res.body = 'hello'
+      assert.strictEqual(5, res.length)
+      res.body = new Response('different body')
+      assert.strictEqual(undefined, res.header['content-length'])
+    })
   })
 })
diff --git a/lib/response.js b/lib/response.js
index 846a8beed..e1aa2b03e 100644
--- a/lib/response.js
+++ b/lib/response.js
@@ -201,6 +201,7 @@ module.exports = {
     // ReadableStream
     if (val instanceof ReadableStream) {
       if (setType) this.type = 'bin'
+      if (original != null) this.remove('Content-Length')
       cleanupPreviousStream()
       return
     }
@@ -217,6 +218,7 @@ module.exports = {
     if (val instanceof Response) {
       this.status = val.status
       if (setType) this.type = 'bin'
+      if (original != null) this.remove('Content-Length')
       const headers = val.headers
       for (const key of headers.keys()) {
         this.set(key, headers.get(key))