"System.ArgumentOutOfRangeException: Index was out of range" when decompiling HL2 Beta maps

[Equalizer5118]

I am attempting to decompile maps from "The Thing", which was the first custom campaign for Half-Life 2 built off the leaked beta code, but after attempting to decompile the maps it keeps throwing this error:

Exception caught in Job 7: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
Parameter name: index
   at System.ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument argument, ExceptionResource resource)
   at Decompiler.BSPExtensions.GetLeavesInTree(BSP bsp, Node node)
   at Decompiler.BSPExtensions.GetBrushesInModel(BSP bsp, Model model)
   at Decompiler.BSPDecompiler.ProcessEntity(Entity entity)
   at Decompiler.BSPDecompiler.Decompile()
   at Decompiler.Job.Run()

I don't know if I am doing something wrong or if this format is just not supported.