sorting null values in grid

I am applying the sorting for the grid column which have integer values and some records have null values. It is not sorting the records properly. After applying the sorting the columns shows the null records in between the records which have values. In my point of view the null values should come first or last of the field as whole but it mingled with the values records.
I used the setTextFormat("###") for the column for applying the integer sort. But it is not working.

You can customize null/error value processing with setErrorText and setErrorValue methods -

http://www.activewidgets.com/aw.system.format/seterrorvalue.html
http://www.activewidgets.com/aw.system.format/seterrortext.html

Custom error value affects the item position after sort (try 0 or Infinity). Custom error text defines what the end user will see.

I tried but it is affecting. I found that the field having empty value.