Having trouble using ISM to create a new RAM model

I've tried to create a new RAM Model from three separate ISM repositories; each fails right after creating storeys with the error:

Value cannot be null.
Parameter name: key    at System.ThrowHelper.ThrowArgumentNullException(ExceptionArgument argument)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at Bentley.IsmRssLink.IModelExtensionMethods.ReinitISMIdStoryDictionary(IModel rssModel)
   at Bentley.IsmRssLink.IsmHandlers.CurveMemberHandler.PreProcess()
   at System.Collections.Generic.List`1.ForEach(Action`1 action)
   at System.Collections.Generic.List`1.ForEach(Action`1 action)
   at Bentley.IsmRssLink.IsmRssLink._CreateFromRepository()

I've created some of these ISM Repos from Revit, and some from RAM using Create ISM Repository, starting with valid, working RAM models. I get the same error each time, and then the progress bar sticks around without advancing anymore. Eventually I closed the window and tried again.

Anybody know what's up?