runtime error 6 overflow??

I get a runtime error 6 overflow at the "FOR" line. It's highlighted yellow when I get the debug error. What is wrong?

For i = 2 To tbox + 1

 aa = CheckSum(xxx)

 Cells(i, "A").Value = txt
 Cells(i, "B").Value = aa

  Cells(i, "C").Value = txt & aa

 
 
 
 
 txt = txt + 1

Next i