Error when adding Conditional Columns - We cannot convert the value null to type Logical.

I am dealing with a client's contact list excel file from Marketo, and have appended additional contacts to the file in the same format as the client's list - I did this in the original Excel. In query editor I am trying to add conditional columns based on campaign names - "If Campaign Name=X, Campaign Description = Y." The Campaign Name in the new records I appended to the excel file are identical to the ones that were already in the client's file. But in the rows containing my additional contacts, I am getting this error in the new conditional column field: 

Expression.Error: We cannot convert the value null to type Logical.
Details:
Value=
Type=Type

What is causing this error and how can I resolve it?