vba error handling

hi!

have code that's something like:

For a = (3 + r - 1) To (21 + r - 1)

cntry = Cells(a, 3).Value
On Error GoTo bob
rst.Open "something something"
   temp = rst!subs
   
         
   calc = final * temp
   
   Cells(a, 25).Value = calc
   

bob:  rst.Close
   Next a

works for a bit and they says, invalid use of null... why is this happening, why does this not trigger the error handler?