If a variable is nil, then it needs to be added to the table as an empty string and not nil. If it's nil, it affects everything that touches it downstream.

When the player loads in, incomplete items will be assigned placeholder values until the player loots the item again.

In the case of a map, Orgrimmar (Horde) or Stormwind City (Alliance) will be used. A source will be added as an empty string, which will be interpreted as Unknown in a tooltip or - in the search output.

There's also a function called Check that should prevent items being added with nil values. If a value is nil, it should be assigned an empty string.