diff --git a/lib/match_body.js b/lib/match_body.js
index 436b7b53a..3d568ecbd 100644
--- a/lib/match_body.js
+++ b/lib/match_body.js
@@ -63,10 +63,11 @@ module.exports = function matchBody(options, spec, body) {
 
 function mapValues(object, cb) {
   const keys = Object.keys(object)
+  const clonedObject = { ...object }
   for (const key of keys) {
-    object[key] = cb(object[key], key, object)
+    clonedObject[key] = cb(clonedObject[key], key, clonedObject)
   }
-  return object
+  return clonedObject
 }
 
 /**
diff --git a/tests/got/test_reply_body.js b/tests/got/test_reply_body.js
index 71f44ff7c..ebedb07af 100644
--- a/tests/got/test_reply_body.js
+++ b/tests/got/test_reply_body.js
@@ -115,6 +115,23 @@ describe('`reply()` body', () => {
     expect(body).to.be.a('string').and.equal('')
     scope.done()
   })
+
+  it('does not modify the object used for response', async () => {
+    const patchBody = { number: 1234 }
+    const form = new FormData()
+    form.append('number', 1234)
+
+    const scope = nock('http://example.test')
+      .patch('/', patchBody)
+      .reply(200, patchBody)
+
+    await got.patch('http://example.test/', {
+      form,
+    })
+
+    expect(patchBody.number).to.be.a('number').and.equal(1234)
+    scope.done()
+  })
 })
 
 describe('`reply()` status code', () => {
diff --git a/tests/test_common.js b/tests/test_common.js
index f91ddb474..68d8922dc 100644
--- a/tests/test_common.js
+++ b/tests/test_common.js
@@ -71,6 +71,18 @@ describe('Body Match', () => {
       const result = matchBody({}, { number: 1 }, '{"number": "1"}')
       expect(result).to.equal(false)
     })
+
+    it('should not modify the original spec object', () => {
+      const spec = { number: 1 }
+      matchBody(
+        {
+          headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
+        },
+        spec,
+        '',
+      )
+      expect(spec).to.deep.equal({ number: 1 })
+    })
   })
 })