0 as a primary key value

[jn]

This is a newbie question, but why can't I have an index value of 0 in my
primary key column?

I want agentid to be 0 for the record "UNASSIGNED," but every time I update
the table it gets set to the next autoincrement value.

This is my primary key:
agentid int(11) unsigned PRI NULL auto_increment

I could rewrite my scripts to not use the 0 value from the database, but
that would take some time. Any ideas?

Thanks