0 - 1 of 1 tags for substraction

Hi, i have these 2 columns defined:

WAIT_INTVL INTERVAL HOUR(3) TO SECOND(2)
HOLD_INTVL INTERVAL HOUR(3) TO SECOND(2)

and both are null values

a) select COALESCE(WAIT_INTVL, INTERVAL '0' SECOND) from TableT1
returns     0:00:00.00

b) i can substract (a) with itself

select COALESCE(WAIT_INTVL, INTERVAL '0' SECOND) - COALESCE(WAIT_INTVL, INTERVAL '0' SECOND)
from TableT1
returns     0:00:00.00