diff --git a/package.json b/package.json
index 0834bcc353b64..eae3531a51ad3 100644
--- a/package.json
+++ b/package.json
@@ -146,6 +146,7 @@
     "find-up": "4.1.0",
     "firebase": "7.14.5",
     "flat": "5.0.2",
+    "form-data": "4.0.0",
     "fs-extra": "9.0.0",
     "get-port": "5.1.1",
     "get-port-please": "3.1.1",
diff --git a/packages/next/src/server/app-render/action-handler.ts b/packages/next/src/server/app-render/action-handler.ts
index eafba6c75866d..993d24fc8373f 100644
--- a/packages/next/src/server/app-render/action-handler.ts
+++ b/packages/next/src/server/app-render/action-handler.ts
@@ -417,6 +417,11 @@ export async function handleAction({
             bound = await decodeReply(formData, serverModuleMap)
           } else {
             const action = await decodeAction(formData, serverModuleMap)
+            if (action === null) {
+              return {
+                type: 'not-found',
+              }
+            }
             const actionReturnedState = await action()
             formState = decodeFormState(actionReturnedState, formData)
 
@@ -495,6 +500,11 @@ export async function handleAction({
             })
             const formData = await fakeRequest.formData()
             const action = await decodeAction(formData, serverModuleMap)
+            if (action === null) {
+              return {
+                type: 'not-found',
+              }
+            }
             const actionReturnedState = await action()
             formState = await decodeFormState(actionReturnedState, formData)
 
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 9d96eec514047..048c19416e4aa 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -284,6 +284,9 @@ importers:
       flat:
         specifier: 5.0.2
         version: 5.0.2
+      form-data:
+        specifier: 4.0.0
+        version: 4.0.0
       fs-extra:
         specifier: 9.0.0
         version: 9.0.0
diff --git a/test/e2e/app-dir/actions/app-action.test.ts b/test/e2e/app-dir/actions/app-action.test.ts
index b4698bf084fa1..29efd9108c395 100644
--- a/test/e2e/app-dir/actions/app-action.test.ts
+++ b/test/e2e/app-dir/actions/app-action.test.ts
@@ -409,32 +409,48 @@ createNextDescribe(
       )
     })
 
-    it('should 404 when POSTing an invalid server action', async () => {
-      const res = await next.fetch('/non-existent-route', {
-        method: 'POST',
-        headers: {
-          'content-type': 'application/x-www-form-urlencoded',
-        },
-        body: 'foo=bar',
+    describe('invalid server actions', () => {
+      it('should 404 when POSTing a x-www-form-urlencoded to a non-existent route', async () => {
+        const res = await next.fetch('/non-existent-route', {
+          method: 'POST',
+          headers: {
+            'content-type': 'application/x-www-form-urlencoded',
+          },
+          body: 'foo=bar',
+        })
+
+        expect(res.status).toBe(404)
       })
 
-      expect(res.status).toBe(404)
-    })
+      it('should 404 when POSTing a multipart/form-data to a non-existent route', async () => {
+        // `form-data` must be used with `node-fetch` otherwise the content-type won't be properly
+        // set as multipart/form-data
+        const FormData = require('form-data') as typeof import('form-data')
+        const data = new FormData()
+        data.append('foo', 'bar')
+        const res = await next.fetch('/non-existent-route', {
+          method: 'POST',
+          body: data,
+        })
 
-    it('should log a warning when a server action is not found but an id is provided', async () => {
-      await next.fetch('/server', {
-        method: 'POST',
-        headers: {
-          'content-type': 'application/x-www-form-urlencoded',
-          'next-action': 'abc123',
-        },
-        body: 'foo=bar',
+        expect(res.status).toBe(404)
       })
 
-      await check(
-        () => next.cliOutput,
-        /Failed to find Server Action "abc123". This request might be from an older or newer deployment./
-      )
+      it('should log a warning when a server action is not found but an id is provided', async () => {
+        await next.fetch('/server', {
+          method: 'POST',
+          headers: {
+            'content-type': 'application/x-www-form-urlencoded',
+            'next-action': 'abc123',
+          },
+          body: 'foo=bar',
+        })
+
+        await check(
+          () => next.cliOutput,
+          /Failed to find Server Action "abc123". This request might be from an older or newer deployment./
+        )
+      })
     })
 
     if (isNextStart) {