When a grenade lands directly onto a belt, the game throws this error:

The mod Grenade explosion delay (0.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event grenade_explosion_delay::on_nth_tick(60)
__grenade_explosion_delay__/control.lua:68: attempt to index local 'grenade' (a nil value)
stack traceback:
    __grenade_explosion_delay__/control.lua:68: in function <__grenade_explosion_delay__/control.lua:42>

Having the grenade be moved onto a belt at any point after hitting the ground does not seem to cause any errors.