fdisk : error code 22

[Bruno Costacurta]

I'm trying to change partition type using fdisk but I receive error 22.
What does this error 22 means ?

Reboot (as suggested by the message) doesn't resolve neither update partition 
type.

(...)
fdisk /dev/sda1

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 83

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 22: Invalid 
argument.
The kernel still uses the old table.
The new table will be used at the next reboot.
Syncing disks.
(...)

Thanks for any clue,
-- Bruno