Module Instalation Issue

Hi,
I am a getting pdo issue when install this module kindly resolved this issue asap. Issue given belwo.

Project user_created_by (7.x-1.0) downloaded to /var/www/html/sites/all/modules/user_created_by. [success]
The following extensions will be enabled: user_created_by
Do you really want to continue? (y/n): y
WD php: PDOException: SQLSTATE[42000]: Syntax error or access violation: 1171 All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, [error]
use UNIQUE instead: CREATE TABLE {user_created_by_data} (
`uid` INT unsigned DEFAULT NULL COMMENT 'The users.uid',
`creator_uid` INT unsigned DEFAULT NULL COMMENT 'The users.uid who created the user',
PRIMARY KEY (`uid`)
) ENGINE = InnoDB DEFAULT CHARACTER SET utf8 COMMENT 'The table to store the user uid and creator’s uid.'; Array
(
)
in db_create_table() (line 2776 of /var/www/html/includes/database/database.inc).
Cannot modify header information - headers already sent by (output started at /usr/share/php/drush/includes/output.inc:37) bootstrap.inc:1511 [warning]
PDOException: SQLSTATE[42000]: Syntax error or access violation: 1171 All parts of a PRIMARY KEY must be NOT NULL; if you need NULL in a key, use UNIQUE instead:
CREATE TABLE {user_created_by_data} (
`uid` INT unsigned DEFAULT NULL COMMENT 'The users.uid',
`creator_uid` INT unsigned DEFAULT NULL COMMENT 'The users.uid who created the user',
PRIMARY KEY (`uid`)
) ENGINE = InnoDB DEFAULT CHARACTER SET utf8 COMMENT 'The table to store the user uid and creator’s uid.'; Array
(
)
in db_create_table() (line 2776 of /var/www/html/includes/database/database.inc).
Drush command terminated abnormally due to an unrecoverable error.