Orphan field in signal forms

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.

Why this happens

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

How to fix it

• 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.