diff --git a/packages/next/server/api-utils/node.ts b/packages/next/server/api-utils/node.ts
index 310c0afd12f4246..62e18957f552ba7 100644
--- a/packages/next/server/api-utils/node.ts
+++ b/packages/next/server/api-utils/node.ts
@@ -442,7 +442,7 @@ function sendJson(res: NextApiResponse, jsonBody: any): void {
   res.setHeader('Content-Type', 'application/json; charset=utf-8')
 
   // Use send to handle request
-  res.send(jsonBody)
+  res.send(JSON.stringify(jsonBody))
 }
 
 function isNotValidData(str: string): boolean {
diff --git a/test/integration/api-support/pages/api/json-null.js b/test/integration/api-support/pages/api/json-null.js
new file mode 100644
index 000000000000000..b0bac6f5f9fad52
--- /dev/null
+++ b/test/integration/api-support/pages/api/json-null.js
@@ -0,0 +1,3 @@
+export default (req, res) => {
+  res.json(null)
+}
diff --git a/test/integration/api-support/pages/api/json-string.js b/test/integration/api-support/pages/api/json-string.js
new file mode 100644
index 000000000000000..f8eb8f057157354
--- /dev/null
+++ b/test/integration/api-support/pages/api/json-string.js
@@ -0,0 +1,3 @@
+export default (req, res) => {
+  res.json('Hello world!')
+}
diff --git a/test/integration/api-support/pages/api/undefined.js b/test/integration/api-support/pages/api/json-undefined.js
similarity index 100%
rename from test/integration/api-support/pages/api/undefined.js
rename to test/integration/api-support/pages/api/json-undefined.js
diff --git a/test/integration/api-support/test/index.test.js b/test/integration/api-support/test/index.test.js
index 09df1e904625626..12b22adcc99a06a 100644
--- a/test/integration/api-support/test/index.test.js
+++ b/test/integration/api-support/test/index.test.js
@@ -163,11 +163,27 @@ function runTests(dev = false) {
   })
 
   it('should support undefined response body', async () => {
-    const res = await fetchViaHTTP(appPort, '/api/undefined', null, {})
+    const res = await fetchViaHTTP(appPort, '/api/json-undefined', null, {})
     const body = res.ok ? await res.text() : null
     expect(body).toBe('')
   })
 
+  it('should support string in JSON response body', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/json-string', null, {})
+    const body = res.ok ? await res.text() : null
+    expect(body).toBe('"Hello world!"')
+  })
+
+  it('should support null in JSON response body', async () => {
+    const res = await fetchViaHTTP(appPort, '/api/json-null')
+    const body = res.ok ? await res.json() : 'Not null'
+    expect(res.status).toBe(200)
+    expect(res.headers.get('content-type')).toBe(
+      'application/json; charset=utf-8'
+    )
+    expect(body).toBe(null)
+  })
+
   it('should return error with invalid JSON', async () => {
     const data = await fetchViaHTTP(appPort, '/api/parse', null, {
       method: 'POST',