object_id can't be string / empty

Hi, i’m getting this error when trying to insert my own activity,

WordPress database error Incorrect integer value: '' for column 'object_id' at row 1 for query
		INSERT INTO wp_simple_history
		SET
			date = '2013-09-04 19:11:35',
			action = 'add_simple_history',
			object_type = 'Email',
			object_subtype = '',
			user_id = '1',
			object_id = '',
			object_name = 'Flight',
			action_description = ''
		 made by do_action('wp_ajax_add_simple_history'), call_user_func_array, add_simple_history_callback, simple_history_add

i manage to get it work by changing the database to varchar thou.

http://ww.wp.xz.cn/plugins/simple-history/