Null values are different than just a blank string. They are actual EMPTY spots. You can't do most logical operations with a null value.
For example, if you have a TotalCount variable and it is set to NULL, you can't see if it is greater than zero or less than zero or anything else. It has no value, it is NULL. So you need to check for that before you start working with it.
The syntax, if you had a TotalCount variable, would be:
NoCount = IsNull(TotalCount)
if NoCount = TRUE then
response.write "you must enter the count value first."
end if
