diff --git a/packages/@n8n/task-runner/src/js-task-runner/__tests__/js-task-runner.test.ts b/packages/@n8n/task-runner/src/js-task-runner/__tests__/js-task-runner.test.ts
index 61c19c5cc2..d2dac70778 100644
--- a/packages/@n8n/task-runner/src/js-task-runner/__tests__/js-task-runner.test.ts
+++ b/packages/@n8n/task-runner/src/js-task-runner/__tests__/js-task-runner.test.ts
@@ -1,6 +1,10 @@
 import { DateTime, Duration, Interval } from 'luxon';
-import type { IBinaryData } from 'n8n-workflow';
-import { setGlobalState, type CodeExecutionMode, type IDataObject } from 'n8n-workflow';
+import {
+	type IBinaryData,
+	setGlobalState,
+	type CodeExecutionMode,
+	type IDataObject,
+} from 'n8n-workflow';
 import fs from 'node:fs';
 import { builtinModules } from 'node:module';
 
@@ -26,6 +30,7 @@ import {
 	withPairedItem,
 	wrapIntoJson,
 } from './test-data';
+import { ReservedKeyFoundError } from '../errors/reserved-key-not-found.error';
 
 jest.mock('ws');
 
@@ -805,6 +810,15 @@ describe('JsTaskRunner', () => {
 					}),
 				).rejects.toThrow(ValidationError);
 			});
+
+			it('should throw a ReservedKeyFoundError if there are unknown keys alongside reserved keys', async () => {
+				await expect(
+					executeForAllItems({
+						code: 'return [{json: {b: 1}, objectId: "123"}]',
+						inputItems: [{ a: 1 }],
+					}),
+				).rejects.toThrow(ReservedKeyFoundError);
+			});
 		});
 
 		it('should return static items', async () => {
diff --git a/packages/@n8n/task-runner/src/js-task-runner/errors/reserved-key-not-found.error.ts b/packages/@n8n/task-runner/src/js-task-runner/errors/reserved-key-not-found.error.ts
new file mode 100644
index 0000000000..2e32466b85
--- /dev/null
+++ b/packages/@n8n/task-runner/src/js-task-runner/errors/reserved-key-not-found.error.ts
@@ -0,0 +1,11 @@
+import { ValidationError } from './validation-error';
+
+export class ReservedKeyFoundError extends ValidationError {
+	constructor(reservedKey: string, itemIndex: number) {
+		super({
+			message: 'Invalid output format',
+			description: `An output item contains the reserved key <code>${reservedKey}</code>. To get around this, please wrap each item in an object, under a key called <code>json</code>. <a href="https://docs.n8n.io/data/data-structure/#data-structure" target="_blank">Example</a>`,
+			itemIndex,
+		});
+	}
+}
diff --git a/packages/@n8n/task-runner/src/js-task-runner/result-validation.ts b/packages/@n8n/task-runner/src/js-task-runner/result-validation.ts
index d5999d1ce4..dd5378a916 100644
--- a/packages/@n8n/task-runner/src/js-task-runner/result-validation.ts
+++ b/packages/@n8n/task-runner/src/js-task-runner/result-validation.ts
@@ -1,6 +1,7 @@
 import { normalizeItems } from 'n8n-core';
 import type { INodeExecutionData } from 'n8n-workflow';
 
+import { ReservedKeyFoundError } from './errors/reserved-key-not-found.error';
 import { ValidationError } from './errors/validation-error';
 import { isObject } from './obj-utils';
 
@@ -19,17 +20,28 @@ export const REQUIRED_N8N_ITEM_KEYS = new Set([
 ]);
 
 function validateTopLevelKeys(item: INodeExecutionData, itemIndex: number) {
-	for (const key in item) {
-		if (Object.prototype.hasOwnProperty.call(item, key)) {
-			if (REQUIRED_N8N_ITEM_KEYS.has(key)) continue;
+	let foundReservedKey: string | null = null;
+	const unknownKeys: string[] = [];
 
-			throw new ValidationError({
-				message: `Unknown top-level item key: ${key}`,
-				description: 'Access the properties of an item under `.json`, e.g. `item.json`',
-				itemIndex,
-			});
+	for (const key in item) {
+		if (!Object.prototype.hasOwnProperty.call(item, key)) continue;
+
+		if (REQUIRED_N8N_ITEM_KEYS.has(key)) {
+			foundReservedKey ??= key;
+		} else {
+			unknownKeys.push(key);
 		}
 	}
+
+	if (unknownKeys.length > 0) {
+		if (foundReservedKey) throw new ReservedKeyFoundError(foundReservedKey, itemIndex);
+
+		throw new ValidationError({
+			message: `Unknown top-level item key: ${unknownKeys[0]}`,
+			description: 'Access the properties of an item under `.json`, e.g. `item.json`',
+			itemIndex,
+		});
+	}
 }
 
 function validateItem({ json, binary }: INodeExecutionData, itemIndex: number) {
diff --git a/packages/nodes-base/nodes/Code/Sandbox.ts b/packages/nodes-base/nodes/Code/Sandbox.ts
index b3e3057d23..0cfa4fb534 100644
--- a/packages/nodes-base/nodes/Code/Sandbox.ts
+++ b/packages/nodes-base/nodes/Code/Sandbox.ts
@@ -185,13 +185,37 @@ export abstract class Sandbox extends EventEmitter {
 	}
 
 	private validateTopLevelKeys(item: INodeExecutionData, itemIndex: number) {
-		Object.keys(item).forEach((key) => {
-			if (REQUIRED_N8N_ITEM_KEYS.has(key)) return;
+		let foundReservedKey: string | null = null;
+		const unknownKeys: string[] = [];
+
+		for (const key in item) {
+			if (!Object.prototype.hasOwnProperty.call(item, key)) continue;
+
+			if (REQUIRED_N8N_ITEM_KEYS.has(key)) {
+				foundReservedKey ??= key;
+			} else {
+				unknownKeys.push(key);
+			}
+		}
+
+		if (unknownKeys.length > 0) {
+			if (foundReservedKey) throw new ReservedKeyFoundError(foundReservedKey, itemIndex);
+
 			throw new ValidationError({
-				message: `Unknown top-level item key: ${key}`,
+				message: `Unknown top-level item key: ${unknownKeys[0]}`,
 				description: 'Access the properties of an item under `.json`, e.g. `item.json`',
 				itemIndex,
 			});
+		}
+	}
+}
+
+class ReservedKeyFoundError extends ValidationError {
+	constructor(reservedKey: string, itemIndex: number) {
+		super({
+			message: 'Invalid output format',
+			description: `An output item contains the reserved key <code>${reservedKey}</code>. To get around this, please wrap each item in an object, under a key called <code>json</code>. <a href="https://docs.n8n.io/data/data-structure/#data-structure" target="_blank">Example</a>`,
+			itemIndex,
 		});
 	}
 }