Receiving error code in account review/revise after creating UDC Cat Code #21 with blank unassigned

I've created UDC Cate Code #21 in accounts for financial statements with a blank category for those accounts which will remain unclassified. When in Review/Revise Accounts to add this code to certain BU accounts, I get an error code that Code 21 is missing on all other BU's. If I create a set of Cat Codes, how do you set up a blank to allow for no classification? If I cannot do this, I have to assign a code for every single account in my database which seems unnecessary. I only need to classify certain BU accounts with these codes.