Create table results in (errno: 121)

[Tom Brown]

Hi,

I'm trying to create a table with the following syntax

CREATE TABLE TNSession
(
SessionID int(11) NOT NULL auto_increment,
IPAddress varchar(50) default NULL,
Created datetime default NULL,
Expired tinyint(4) default '0',
AccountID int(11) default '0',
PRIMARY KEY (SessionID),
KEY FK_AccountID (AccountID),
CONSTRAINT `0_20` FOREIGN KEY (`AccountID`) REFERENCES `TNAccount`
(`AccountID`)
) TYPE=InnoDB ROW_FORMAT=DYNAMIC;

it results in a

ERROR 1005: Can't create table './TNMailServer/TNSession.frm' (errno: 121)

This is on version mysql-standard-4.0.18-pc-linux-i686

Can anyone shed any light on this as it has me stumped - There is a
TNAccount table with a AccountID column?

thanks for any assistance

Tom