TYPO3 Core

If I try to save a new record in the inline relation
I'm getting an error:

Doctrine\DBAL\Exception\DriverException: An exception occurred while executing 'UPDATE `sys_category` SET `related_category` = ? WHERE `uid` = ?' with params ["", 304]: Incorrect integer value: '' for column 'related_category' at row 1 in vendor/doctrine/dbal/lib/Doctrine/DBAL/Driver/AbstractMySQLDriver.php on line 115

Steps to reproduce:
- install attached test extension
- then edit sys_category
- add an inline child - in the related category field
- try to save it

TCA for the related category field looks like:

'related_category' => array(
    'label' => 'Related category (1. add relation, 2. "Parent" category tree is missing)',
    'config' => array(
      'type' => 'inline',
      'foreign_table' => 'sys_category',
    ),