Whenever I try to place the second piece of a fluid elevator, the game crashes with the following message:

Error while running event Subsurface::on_built_entity (ID 6)
Subsurface/scripts/elevators.lua:34: attempt to index field '?' (a nil value)
stack traceback:
Subsurface/scripts/elevators.lua:34: in function 'switch_elevator'
Subsurface/scripts/elevators.lua:202: in function 'elevator_built'
Subsurface/control.lua:582: in function <Subsurface/control.lua:556>

A similar crash (elevators.lua, line 34) occurs if I try having a construction bot place the fluid elevator instead. Not sure if there's any workaround I can use in the meantime.

This crash does not occur if I roll back to version 1.1.18. Possibly one of the bugfix changes in 1.1.19 introduced this?

Does it work with 1.1.19?
It is fixed for the next release, you can find the working version of the file here: https://github.com/NathaU/factorio-subsurface/blob/master/scripts/elevators.lua

Awesome; thanks!