I get an error on the 'add' line of the following code:

With rng.Validation
.Delete
.Add Type:=xlValidateDecimal, AlertStyle:=xlValidAlertStop,
Operator:=xlGreater, Formula1:="-9999999999999999"
.IgnoreBlank = True
.InCellDropdown = True
.InputTitle = ""
.ErrorTitle = "Error"
.InputMessage = ""
.ErrorMessage = "Must be an Amount."
.ShowInput = True
.ShowError = True
End With

I set rng to a range on the worksheet. What can I look for that would be
causing this error?
Thanks