SELECT subtraction issue

[Andy CLyde]

I have just come across an odd issue when migrating from 3.23.58 on RHEL3 to 4.1.12 on RHEL4.
I had a query in my application like this:
SELECT (field1-field2-1) as new_stock FROM table WHERE table_id=XXX
where
field1 is SMALLINT(6)
field2 is TINYINT(3) UNSIGNED
This was working fine on the old server but on the new server, it's throwing up odd results where the result of (field1-field2-1) is negative.
For example, this query:
SELECT (field1-field2-1) as new_stock FROM table WHERE table_id=8477
(field1 and field2 are both zero in the database)
returns
new_stock = 18446744073709551615

Is this a bug? Or did something change from v3.23.58 to v4.1.12 that I haven't spotted that is causing this?
Thanks
Andy