How can I do set NULL into the table in Doctrine

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I have some table \[code\]store_section\[/code\](\[code\]id\[/code\],\[code\]parent_id\[/code\],\[code\]label\[/code\]), I want to change some row, set parent_id=null.I trying to:\[code\]$record = $table->getTable()->find( $id );$record->parent_id = null;$record->save();\[/code\]But this isn't work. How can I do set NULL into the table in Doctrine, in example above, parent_id becomes =0 (not =NULL)?Thnx for the responses!