# MongoServerError: Unknown modifier / unknown top level operator: $foo / Cannot use $in with a non-array

**Code:** `Query failed: $or / $in / projection errors (BadValue)` · **Category:** MongoDB · **Seen on:** MongoDB

The query or update used an invalid operator or argument type.

## Meaning

The message names the bad operator. Fix typos (`$inc` vs `$incr`), ensure `$in` gets an array, don't mix inclusion and exclusion projections, wrap update fields in operators (`$set`), and don't update `_id`. Test the query in mongosh first.

## Common causes

- Misspelled operator (`$gte`, `$set`)
- `$in`/`$nin` with non-array
- Update document without `$set` mixing operators and fields
- Mixing include/exclude projection
- Modifying `_id`

## Quick fix

1. Check the exact operator name and argument type
2. Use `{ $set: { field: value } }` for updates
3. Use only inclusion or only exclusion in projections

## Detailed fix by platform

### mongosh

1. `db.users.updateOne({ _id }, { $set: { name: "A" } })`

## How to diagnose

1. **Operator** — Valid name?
2. **Type** — Array where needed?
3. **Update** — Uses $set?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/mongodb/sort-operator-field-not-found-or-too-many-fields)