Minetest v5.1.0: Attempt to unregister non-existent element - 'underground'

[redblade7]

After upgrading to v5.1.0, I can't start minetestserver anymore on a server with FaceDeer's fork of caverealms and its dependencies as I crash with this error:

2019-10-18 18:51:31: ERROR[Main]: ModError: Failed to load and run script from (REMOVED)/bin/../games/minetest_game/mods/caverealms/init.lua:
2019-10-18 18:51:31: ERROR[Main]: (REMOVED)/bin/../games/minetest_game/mods/subterrane/legacy.lua:496: Attempt to unregister non-existent element - 'underground'
2019-10-18 18:51:31: ERROR[Main]: stack traceback:
2019-10-18 18:51:31: ERROR[Main]: [C]: in function 'error'
2019-10-18 18:51:31: ERROR[Main]: (REMOVED)/bin/../builtin/game/register.lua:532: in function 'unregister_biome'
2019-10-18 18:51:31: ERROR[Main]: (REMOVED)/bin/../games/minetest_game/mods/subterrane/legacy.lua:496: in function 'override_biome'
2019-10-18 18:51:31: ERROR[Main]: ...ames/minetest_game/mods/caverealms/caverealms_biomes.lua:527: in main chunk
2019-10-18 18:51:31: ERROR[Main]: [C]: in function 'dofile'
2019-10-18 18:51:31: ERROR[Main]: (REMOVED)/bin/../games/minetest_game/mods/caverealms/init.lua:17: in main chunk

I was able to get minetestserver to start by commenting out subterrane:override_biome() (lines 527 through 533) in caverealms_biomes.lua. I flew through the map on noclip and full range mode and I didn't have any crashes or notice any odd behavior. But please let me know if I'm wrong.