CastError: Cast to ObjectId failed for value "x" (type string) at path "_id"
A value that isn’t a valid 24-hex-character ObjectId was used in an _id query.
Exact message you may see
Cast to ObjectId failed for value
Seen on:
Node.js
Meaning
Routes like /users/:id receive anything; strings like "undefined", "me" or route collisions (/users/new hitting /users/:id) fail casting.
Common causes
- Invalid id from the URL
- Route order: /users/new matched by /users/:id
- "undefined" string from the frontend
⚡ Quick fix
- Validate with mongoose.isValidObjectId(id) and return 400/404
- Register specific routes before parameter routes
Detailed fix by platform
Node.js
if (!mongoose.isValidObjectId(req.params.id)) return res.status(400).json({ error: 'invalid id' });
How to diagnose
- Value — Which value?
- Route — Which route matched?
🔧 Still not fixed?
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Last updated 13 Oct 2026