Can't do anything besides place decoration and furniture in custom build

You can’t place anything in the custom build thing besides furniture and decorations.
Steps to reproduce:

  1. Mine stairs into mountain.
  2. Have 2 filthy peasants stand on the stairs.
  3. Try to decorate those stairs with slabs with the peasants in the way.
  4. Struggle as the game will no longer allow you to place anything besides furniture and decorations.

Expected Results:
I would make some fresh looking stairs.
Actual Results:
I can’t place anything but furniture and deco in this world and now I have to start over again.
Here’s what showed up:
release-593 (x64)
stonehearth/lib/build_util.lua:356: assertion failed!
stack traceback:
radiant/modules/common.lua:237: in function 'report_traceback’
radiant/modules/common.lua:248: in function <radiant/modules/common.lua:242>
[C]: in function 'assert’
stonehearth/lib/build_util.lua:356: in function ‘can_start_blueprint’
…earth/components/fabricator/fabricator_component.lua:792: in function ‘_start_project’
…earth/components/fabricator/fabricator_component.lua:572: in function ‘_updates_state’
…earth/components/fabricator/fabricator_component.lua:561: in function 'obj’
radiant/modules/events.lua:75: in function 'instance’
radiant/modules/events.lua:227: in function <radiant/modules/events.lua:221>
[C]: in function 'xpcall’
radiant/modules/common.lua:257: in function 'xpcall’
radiant/modules/events.lua:221: in function 'trigger’
radiant/modules/events.lua:290: in function '_trigger_gameloop’
radiant/modules/events.lua:338: in function '_update’
radiant/server.lua:62: in function <radiant/server.lua:58>
Saved world :
1469756094244 - Google Drive
Version Number and Mods in use:
Latest version, no mods.