How to to add a field to a search using a lookup table?

[adent]

I am trying to add a field to a search using a lookup table. However, my key field  is sometimes blank and I get an error the lookup table does not exist or is not available. 

... search with an output field of user

| lookup userList.csv id as user OUTPUT title

There are a few lines rows where id is blank and I do not have permissions to edit the table.

[adent]

found one solution is to do the fillnull before the subsearch.  #337963