Access Form Multiple Checkbox's auto-populate the 3rd based off the 1st and 2nd Checkbox

[Mac1206]

I have 3 Checkboxes on my form and I'm trying to make the third checkbox automatically populate if the first 2 are checked, see below...Any suggestions on how this can be completed? For the Third checkbox Field C1 on my Form, I need it to auto-populate with True or Yes or -1 based off the 1st 2 being checked and if one of the 1st 2 is not then leave it blank...

Does this need to be in the Control Source, Default Value, or an expression built into the query for the form in order to get Field C1 to populate to -1?

IIf([A1]="Yes" And [B1]="Yes",[C1]=-1,"")

 

[Micron]

You could always put in breakpoints in the code
F9 where your cursor is, then step through

Why, in this case? I don't think either suggestion has been implemented yet, so there's no code to check, and as of yet, no error to look for.
Re post 11, Date() is required in sql; Date in vba.
Although not really the same, people get confused over IsNull in vba, Is Null in sql as well. IsNull is a function. Is is an operator checking for Null.