n8n error index

Code node · Code

A 'json' property isn't an object

You returned the right envelope — an item with a json key — but json points at something that isn't a plain object. n8n's description:

In the returned data, every key named 'json' must point to an object.

The identical check runs on binary, and produces A 'binary' property isn't an object. Everything below applies to both.

What n8n counts as "an object"

This is stricter than JavaScript's typeof, and the exact test is four conditions:

typeof value === 'object' && value !== null && !Array.isArray(value) && !(value instanceof Date)

So all four of these fail, and three of them surprise people:

What you returned	Why it fails
json: null	typeof null === 'object', but n8n excludes it explicitly
json: [ … ]	arrays are objects to JavaScript; n8n excludes them
json: new Date()	a Date is an object; n8n excludes it
json: 'some string', json: 42, json: undefined	not objects at all

The array case is the common one. { json: results } where results is an array of rows reads perfectly and is wrong — each row needs to be its own item:

// throws
return [{ json: rows }];

// right
return rows.map(row => ({ json: row }));

The Date case is the sharp one, because a Date inside json is fine. n8n walks the contents of json and stringifies anything that isn't a plain object — Dates, RegExps and the like — at any depth. So { json: { created: new Date() } } works and comes out as a string, while { json: new Date() } throws. The cleanup applies to what is inside the envelope, never to the envelope itself.

The undefined case, which looks like nothing at all

{ json: undefined } throws, and so does an item where a branch of your code simply never set json:

return $input.all().map(item => {
  const out = {};
  if (item.json.active) out.json = { id: item.json.id };
  return out;          // inactive items have no json at all
});

That's the same failure as json: undefined, and it's the one that shows up on item 40 of 50 rather than on the first item — because it's data-dependent. If this error names a late item index, look for a conditional that doesn't assign on every path.

return null is a different problem

Returning null as your whole result doesn't produce this message. It gets past the earlier shape checks (typeof null === 'object') and then fails inside n8n's item normalisation with a plain JavaScript type error rather than a validation message. If you're seeing a type error with no helpful description, check whether some path returns null instead of [].

Reading the bracketed suffix

• A 'json' property isn't an object [item 4] — item 4 specifically
• … [line 12, for item 4] — when n8n knows the line as well

Item 0 failing usually means the shape is wrong for every item. A late index almost always means a data-dependent branch, as above.

Why the envelope has to be an object

The json object is what the rest of n8n operates on: expressions resolve against its keys, the table view renders its columns, and downstream nodes address fields by name. An array has indices rather than names, a Date has neither, and null has nothing at all. None of them can carry the field access that every node after this one assumes.

Validating at the boundary is deliberate — the alternative is a workflow that runs another three nodes and then fails with an expression error pointing at code you didn't write.

Related errors

• Invalid output format — your item mixes a reserved key with one of your own.
• Unknown top-level item key — your array is inconsistent: some items are n8n items, some aren't.
• Code doesn't return items properly — the outer shape is wrong; the return value isn't an array of objects.

---

Source: validateItem and isObject in packages/nodes-base/nodes/Code/result-validation.ts and utils.ts, and standardizeOutput in the same utils.ts for the stringify-inside-json behaviour. Read directly from n8n's repository; the Code node documentation does not list these error strings.

Related errors on this site

• Invalid output format
• Unknown top-level item key
• Code doesn't return items properly

Last verified 16 September 2026 against n8n 2.39.5 (source read on master @ 2.40.0). n8n changes these messages between releases; if the wording you see differs, the version above is what this page was checked against.