Tab over not working correctly

Hello,

I have the following three rows

CREDITRow1 has the following custom validation script:   this.getField("DEBITRow1").readonly = (Number(event.value)>0);

If I enter a value in DEBITRow1 it correctly make CREDITRow1 read only, but when I go to tab from DEBITRow1 to Line DescriptionRow1 it tabs back to the top of the page and the first field. How can I correct this?

Thank you