diff --git a/packages/next/next-server/server/image-optimizer.ts b/packages/next/next-server/server/image-optimizer.ts
index 69bb10f261ca873721f0d98c6fe2e84a814c20f1..eef496c15f6a4629a60cb87b67162b5fb783e158 100644
--- a/packages/next/next-server/server/image-optimizer.ts
+++ b/packages/next/next-server/server/image-optimizer.ts
@@ -230,6 +230,13 @@ export async function imageOptimizer(
       sendResponse(req, res, upstreamType, upstreamBuffer)
       return { finished: true }
     }
+
+    // If upstream type is not a valid image type, return 400 error.
+    if (!upstreamType.startsWith('image/')) {
+      res.statusCode = 400
+      res.end("The requested resource isn't a valid image.")
+      return { finished: true }
+    }
   }
 
   let contentType: string
diff --git a/test/integration/image-optimizer/public/text.txt b/test/integration/image-optimizer/public/text.txt
new file mode 100644
index 0000000000000000000000000000000000000000..32f95c0d1244a78b2be1bab8de17906fabb2c4a8
--- /dev/null
+++ b/test/integration/image-optimizer/public/text.txt
@@ -0,0 +1 @@
+hi
\ No newline at end of file
diff --git a/test/integration/image-optimizer/test/index.test.js b/test/integration/image-optimizer/test/index.test.js
index 41cb945cb3767e7b9038f1bf215e2957e9887972..f5bf0e2847b9663accec01c08da7b2bf636f6a8b 100644
--- a/test/integration/image-optimizer/test/index.test.js
+++ b/test/integration/image-optimizer/test/index.test.js
@@ -481,6 +481,14 @@ function runTests({ w, isDev, domains }) {
     expect(res.headers.get('etag')).toBeTruthy()
     await expectWidth(res, 400)
   })
+
+  it("should error if the resource isn't a valid image", async () => {
+    const query = { url: '/test.txt', w, q: 80 }
+    const opts = { headers: { accept: 'image/webp' } }
+    const res = await fetchViaHTTP(appPort, '/_next/image', query, opts)
+    expect(res.status).toBe(400)
+    expect(await res.text()).toBe("The requested resource isn't a valid image.")
+  })
 }
 
 describe('Image Optimizer', () => {