Engine Error - 'attempt to index a nil value'

Summary:

Error occurred while adding walls to a building.

Steps to reproduce:

release-696 (x64)
stonehearth/components/wall/wall_component.lua:369: attempt to index a nil value
stack traceback:
[C]: ?
stonehearth/components/wall/wall_component.lua:369: in function 'connect_to_columns’
stonehearth/services/server/build/build_service.lua:1264: in function 'init_fn’
stonehearth/services/server/build/build_service.lua:529: in function '_create_blueprint’
stonehearth/services/server/build/build_service.lua:1262: in function '_create_wall’
stonehearth/services/server/build/build_service.lua:1249: in function '_add_wall_span’
stonehearth/services/server/build/build_service.lua:1078: in function 'visitor_fn’
stonehearth/lib/build_util.lua:819: in function 'grow_walls_around’
stonehearth/services/server/build/build_service.lua:1077: in function 'grow_walls’
stonehearth/services/server/build/build_service.lua:1049: in function 'cb’
stonehearth/services/server/build/build_service.lua:1582: in function 'do_command’
stonehearth/services/server/build/build_service.lua:1048: in function <stonehearth/services/server/build/build_service.lua:1046>

Version Number and Mods in use:
Alpha 20

System Information:
Windows 10