This error indicates that a `Field` instance is no longer connected to the current model structure.

Angular signal forms throw `NG01902` when a field path that previously existed can no longer be resolved as the same property in the parent object.

A common trigger is unintentionally setting a field's value to `undefined`.

In signal forms, `undefined` means "field does not exist", so an existing field can become orphaned.

Signal forms keep field instances in sync with your model shape.

If code still references an older field instance after the model shape changed, Angular detects the mismatch and throws this error.

This can happen when:

- a property is removed from the model object

- a property becomes `undefined` instead of remaining present

- code holds a stale field reference across structural updates

- Avoid writing `undefined` to model fields. Use `null` or another explicit value when the field should still exist.

- Keep the model shape stable while a field is in use.

- Re-read fields from the current form tree after structural updates instead of reusing stale references.

For model design guidance, see [Avoid `undefined`](/guide/forms/signals/model-design#avoid-undefined).