need help please

hi, i have a lil' script and is not working and i wish to know what i have done wrong please

tha script:

for m in meditmaterials[2] do
(
if m.opacityMap != undefined do
(
m.opacityMap = undefined
)
)

and i've got this

-- Error occurred in m loop
-- Frame:
-- m: undefined
-- Unknown property: "opacityMap" in undefined
OK

and sorry if i post this here but i don;t know where

thanks in advance