Text box has no value but is not null?

[ARW17]

I have an equation on a subform to add up the number of vacation days scheduled. The equation is
=Sum([Duration by Day])

It works if the person has a vacation day scheduled, but if they don't I'm getting #Error.

I tried to fix it by writing =iif(isnull(Sum([Duration by Day]))=true, 0, Sum([Duration by Day]), but this still results in an error.

How can this text box have no value, but not be null?

Please help me fix this error

 

[ARW17]

I took your advice. It seems more like a workaround than a solution, but I like the result.

Thanks for your help in getting to the bottom of my issue! It does look cleaner for my form to only show the subforms with data.

 

[Joe4]

I took your advice. It seems more like a workaround than a solution, but I like the result.

I don't know if I would call it a workaround. That is usually how you want to program things - only show objects/sections that have data.
You cannot run a sum on data that does not exist. The error results because there is no data set. I don't think you can really tell the formula to ignore that fact.