please consider the following formula, it returns a 'Yes' or 'No' from a Vlookup return depending if the record is a string 'True' or 'False'

=IF(VLOOKUP(A5,'Z:\Information Analysis\CSP Study Log Development\CSP Reporting\Excel Solution\[CSPProject.xls]Sheet1'!A:N,13,FALSE),"YES","NO")

However i have noticed that there are some blank cells in the data in which it is still returned as a 'No'. Can anyone please advise how to alter this to return a 'Not Assigned' if the cell is blank?

many thanks