If cells in formula are blank, then leave the formula cell blank

I have three main columns: Planned Start Date, Planned End Date, and Date Completed

I am utilizing the formula below to determine which tasks are done and late in the status column. I would also like the status cell to be empty if the date completed cell is empty. How can I do that? For some reason ISBLANK isn't working for me. 

=(IF([Date Completed]12 <= [Planned End Date]12, "Done", IF([Date Completed]12 > [Planned End Date]12, "Late")))


Help Article Resources

Want to practice working with formulas directly in Smartsheet?

Check out the Formula Handbook template!