my code works corectly but gives this error.

runtime error: Undefined operation
proc name: calculate (/obj/ship/proc/calculate)
usr: the jobe (/mob)
src: Z-5000 Immortal (/obj/ship)
call stack:
Z-5000 Immortal (/obj/ship): calculate()

it gives the correct output and dose what it is supposed to.. i cant see why it would do this..
This usually happens when you try to do arithmetical operations on things that aren't numbers. Like "5 + /obj".