Days late formula returning 4139 in date field

Hi all I am a newbie on here and finding my way around excel. My business create quotes and these quotes have allocated times for completion depending on their difficulty. I have three types of quotes and three seperate allocation of days to do them.

I have formula's to get the number of days late =TODAY()-H3 with H3 being the due date. however when I drag the formula down as soon as it hits a cell on row H without a date (ie when there is not a quote logged on that line) it automatically returns a 4139 number in the field.??

is there any better way to do this. I basically need to log how many quotes are behind schedule by counting which due date is less than today?? I'm sure I'm just running around the block here rather than going in a straight line..Matt Tracker (DO NOT USE).xlsx

any help would be much appreciated

Kind Regards

Marcus

two days thats taken me and you solve it in half an hour!!

Awesome thank you so much!!!

Marcus