Integer Overflow

Why do we get an integer overflow with this code:

select
cast(1.000 as numeric(18,10)) * 1.0000 * 1.000 * 1.0 * 1.0 from
rdb$database



but not this one?

select
cast(1.000 as numeric(18,10)) * 1.0000 * 1.0 * 1.0 * 1.0 from
rdb$database


Never mind why I would want to do anything as pointless as multiplying lots of ones together, but a more complex calculation reduced to this should not crash.
Firebird 2.1.1.17910 on a number of windows systems
and Firebird 2.1.0.17798

This is a SERIOUS BUG!! Help!!