diff --git a/packages/next/server/api-utils.ts b/packages/next/server/api-utils.ts
index 510d3019662c..42b37536fc38 100644
--- a/packages/next/server/api-utils.ts
+++ b/packages/next/server/api-utils.ts
@@ -69,12 +69,12 @@ export async function apiResolver(
     const writeData = apiRes.write
     const endResponse = apiRes.end
     apiRes.write = (...args: any[2]) => {
-      contentLength += Buffer.byteLength(args[0])
+      contentLength += Buffer.byteLength(args[0] || '')
       return writeData.apply(apiRes, args)
     }
     apiRes.end = (...args: any[2]) => {
       if (args.length && typeof args[0] !== 'function') {
-        contentLength += Buffer.byteLength(args[0])
+        contentLength += Buffer.byteLength(args[0] || '')
       }
 
       if (contentLength >= 4 * 1024 * 1024) {
diff --git a/test/integration/api-support/pages/api/nullable-payload.js b/test/integration/api-support/pages/api/nullable-payload.js
new file mode 100644
index 000000000000..7e9dd82f2186
--- /dev/null
+++ b/test/integration/api-support/pages/api/nullable-payload.js
@@ -0,0 +1,4 @@
+export default (req, res) => {
+  res.write('')
+  res.end()
+}
diff --git a/test/integration/api-support/test/index.test.js b/test/integration/api-support/test/index.test.js
index 266c6eb46065..21a5874de3ad 100644
--- a/test/integration/api-support/test/index.test.js
+++ b/test/integration/api-support/test/index.test.js
@@ -442,6 +442,11 @@ function runTests(dev = false) {
     expect(data).toBe('hi')
   })
 
+  it('should work with nullable payload', async () => {
+    const data = await renderViaHTTP(appPort, '/api/nullable-payload')
+    expect(data).toBe('')
+  })
+
   it('should warn if response body is larger than 4MB', async () => {
     let res = await fetchViaHTTP(appPort, '/api/large-response')
     expect(res.ok).toBeTruthy()