Error from workflow to update a list when value in another list changes

Question

I created a workflow to update a list (Rooms) when another list (People) is changed. I want to assign a value in the Room column in People list and have the value in column LastName be added to the Occupant column in Rooms. I've tried several iterations, and keep getting an error that has no meaningful information (at least to me).

Here's my workflow:

Here's the People list settings:

Here's the Room list settings:

Here's the workflow error (Status: suspended):

Any idea what this error means or what I'm doing wrong? What value can't be null? Isn't that what I'm checking for in the first step? As a first-time Sharepoint user, I'm really discouraged by how difficult this simple task seems to be.

Answer 1

I'm making some assumptions here, but is the Room in the People list unique, i.e. are their multiple list entries in the people list that have the same room? If so the workflow will not be able to use room in the people list as a unique identifier in the lookup, since there is more than one entree in the people list with the same room.

Answer 2

In transition to stage, try to remove the condition and just go to the end of workflow. See if the error will still exist.