Sorting number column - empty cells count as zero

I've been using this to set the column to a number format:

var myData = [
["file1", "40,000 KB"],
["file2", "500 KB"],
["file3", "0 KB"],
["folder1", ""]
["folder2", ""]
]

obj.setCellText(myData);
var string = new AW.Formats.String;
var number = new AW.Formats.Number;
obj.setCellFormat([string, number]);

Now, the sorting works well - the numbers (file sizes in KB) are sorted as they should BUT completely empty cells, or cells with just a string, are sorted as if they have the number 0 (zero) in them.

I wish for completely empty cells to be sorted seperately then the cells with 0 in them.
Please help :)
Thanks