PhpMyAdmin ERROR

Hello, I have a problem with PhpMyAdmin... He send me that : Warning in ./libraries/plugins/auth/AuthenticationCookie.php#449 ]http://localhost/phpmyadmin/themes/dot.gif
hash_equals(): Expected user_string to be a string, bool given Backtrace ./libraries/plugins/auth/AuthenticationCookie.php#449: hash_equals( string 'root', boolean false, ) ./libraries/common.inc.php#756: PMA\libraries\plugins\auth\AuthenticationCookie->authSetUser() ./index.php#13: require_once(./libraries/common.inc.php) Warning in ./libraries/plugins/auth/AuthenticationCookie.php#456 ]http://localhost/phpmyadmin/themes/dot.gif
hash_equals(): Expected user_string to be a string, bool given Backtrace ./libraries/plugins/auth/AuthenticationCookie.php#456: hash_equals( string 'root', boolean false, ) ./libraries/common.inc.php#756: PMA\libraries\plugins\auth\AuthenticationCookie->authSetUser() ./index.php#13: require_once(./libraries/common.inc.php)
I hope that you can help me ... Thanks