vbscript (for each x in request.form) ignores empty fields

I want to update my database according to retrieved form fields where empty fields should set database field ="not present". I used this loop through form fields but it seems that empty fields is not listed in my loop at all and so the linked field remains unchanged. What is the alternative solution?

(in this case my fields are input with type="checkbox")

for each x in request.form
 if request(x)<>"" then 
  rs(x)=request(x)
  else
  rs(x)="not present"
 end if
next