[5.1->5.5] mysql_fix_privilege_tables error 1064

[Vincent Duvernet]

Hi,
I'm trying to upgrade from 5.1.73 to 5.5.59 and followed the tutorial :
http://myworldofit.net/?p=9623

but running following command leads to some problems :
Command :
---------
mysql_upgrade.exe -u root -p --verbose

Result :
--------
[...]

Running 'mysql_fix_privilege_tables'...
ERROR 1064 (42000) at line 47: You have an error in your SQL syntax; check the m
anual that corresponds to your MySQL server version for the right syntax to use
near '(14), Table_priv set('Select','Insert','Update','Delete','Create','Drop','
Grant'' at line 1
ERROR 1064 (42000) at line 49: You have an error in your SQL syntax; check the m
anual that corresponds to your MySQL server version for the right syntax to use
near '(14), Column_priv set('Select','Insert','Update','References') COLLATE utf
8_gene' at line 1
ERROR 1064 (42000) at line 81: You have an error in your SQL syntax; check the m
anual that corresponds to your MySQL server version for the right syntax to use
near '(14), PRIMARY KEY (Host,Db,User,Routine_name,Routine_type), KEY Grantor (G
rantor' at line 1
ERROR 1064 (42000) at line 483: You have an error in your SQL syntax; check the
manual that corresponds to your MySQL server version for the right syntax to use
near '(14) AFTER Proc_priv' at line 2
FATAL ERROR: Upgrade failed

The file 'mysql_fix_privilege_tables' is present in folder 'C:\Program Files\MySQL\MySQL Server 5.1\share\
but not in 'C:\Program Files\MySQL\MySQL Server 5.5\share'

My wordpress site seems to be running using 5.5 version without any problem.
(and I want to upgrade to the latest MySQL version... :/)

Thanks for help.
Vincent