Why is a Procedural Foliage Spawner Crashing the Editor When I Drag It In?

I created a new procedural foliage spawner in the content creator, but haven’t added any foliage to it yet. When I drag it into the world, the editor crashes, and I receive the following error:

Fatal error: [File:/Users/build/Build/++UE4/Sync/Engine/Source/Runtime/CoreUObject/Private/Templates/Casts.cpp] [Line: 10] Cast of LightmassCharacterIndirectDetailVolume /Game/My_Level/Landmass_Map.Landmass_Map:PersistentLevel.LightmassCharacterIndirectDetailVolume_0 to ProceduralFoliageVolume failed

Any help to get this figured out would be greatly appreciated! Painting trees by hand is tedious…

Thank you!