Have you tried with
isnull(field) or field=null() or len(trim(field)) = 0 (or trim(field) = '')
of course 3 and 4 are the same. Remember that null() and isnull(field) is not the same.
The actual lenght of the field is not working properly, but maybe you need to post the script and expression.
A database char field can contain blanks which is not the same as database null. Additionally, there have been issues with consistency of IsNull() on the 64bit platform. There are also differences between blank trimming on database reads and file reads (controlled by Verbatim= variable). I've found the most reliable way to detect "empty" fields for all cases is
Although I will admit that your "z" example looks strange. I debug these cases with an listbox expression of
='>' & field & '<'
What expression are you using for your "Length of Sales Deal Field" field? Is that a calculated dimension or an expression?