# X_MIGRATION_APPEND_ONLY_BACKFILL

## Means

a NOT NULL column added to an append-only table needs a default

## Typical cause

existing rows could only get a value by an UPDATE the append-only trigger refuses

## Fix

`entity('<name>', { columns: { <col>: <builder>.default(<value>) } })` — then re-run x db gen

[Entity and database](/error-codes#entity-and-database)