# DRecIndex::type-mismatch


**Key:** `DRecIndex::type-mismatch`

**Category:** indexing · **Severity:** error

**Source:** [`src/diag/hints.nix:854`](https://github.com/kleisli-io/nix-effects/blob/main/src/diag/hints.nix#L854)

the index position of `rec` must match the Desc's declared index type. Pass a term of that index type, or adjust the enclosing `μ I ...` to match.

## Example

`Hint::DRecIndex::type-mismatch` means the `DRecIndex` position supplies a term whose type does not match the expected type. The example shows the failing form first, then one way to give the checker the missing structure.

```nix
# Bad: the recursive position points at the wrong index type.
descRec wrongIndex tailDesc

# Better: recurse at an index from the Desc index type.
descRec expectedIndex tailDesc

```


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