I have some Conditional formatting in my spreadsheet and can't add another format to my cell so I need to only highlight it red if the value is empty.

Right now I am using the following command to highlight a row red if the dates listed are before a date I placed in B1

=INDIRECT("SHEET1!Q"&ROW())<$B$1

Assigned to column: =$AC$13:$AC$402

It marks all cells before the date red but also marks blank cells in this same column red. I have tried a few different variations but I can't quite get one that will do nothing if blank and mark the rest red.