I want to turn this if formula into an array formula but cant seem to get it to auto populate down the rows

I want the driving distance between two address's and I am running a script that allows the following statement work.

THIS WORKS but I have to drag the formula down when new rows are added from my mobile app into my sheet

=(IF(ISBLANK($AF2:$AF),"",DrivingMeters($E2:$E,$AF2:$AF)/1000))

THIS DOESNT WORK

=ARRAYFORMULA(IF(ISBLANK($AF$2:$AF),"",DrivingMeters($E$2:$E,$AF$2:$AF)/1000))

I cant work out what I am doing wrong?