TYPO3 error: Incorrect integer value: '' for column at row 1

I'm using TYPO3. I programmed an extension called 'Eventmanager', to manage events.

After installing this extension, in Backend I can add/edit event-records. With each record there is a field called end_time, which is a datetime-picker, for choosing the date and time of the events.

Today I edit an event-record. I deleted the value of this end_time field (to make it empty), and tried to save this record. But an error occurred during saving:

2: SQL error: 'Incorrect integer value: '' for column 'event_end_time' at row 1' (tx_eventmanager_domain_model_event:13)

The Settings in TCA is shown as follows:

    'endtime' => array(
        'exclude' => 1,
        'l10n_mode' => 'mergeIfNotBlank',
        'label' => 'LLL:EXT:lang/locallang_general.xlf:LGL.endtime',
        'config' => array(
            'type' => 'input',
            'size' => 13,
            'max' => 20,
            'eval' => 'datetime',
            'checkbox' => 0,
            'default' => 0,
            'range' => array(
                'lower' => mktime(0, 0, 0, date('m'), date('d'), date('Y'))
            ),
        ),
    ),

and the definition in the file 'ext_tables.sql' is shown as follows:

event_end_time int(11) DEFAULT '0' NOT NULL

Can someone tell my what is the reason? How can I solve this problem?