Attach Labels to Points on Scatter Chart

Hi, I am using the following code to assign labels on a scatter chart but am having trouble with one of the lines. I sometimes receive an error on the following line and other times it works fine:
"xVals = ActiveChart.SeriesCollection(1).Formula"
The error message is "Object variable or With Block variable not set"
I was wondering if someone might be able to help by explaining the cause of the problem?

Many thanks.

Sub AttachLabelsToPoints()

   'Dimension variables.
   Dim Counter As Integer, ChartName As String, xVals As String

   ' Disable screen updating while the subroutine is run.
   Application.ScreenUpdating = False

   'Store the formula for the first series in "xVals".
   xVals = ActiveChart.SeriesCollection(1).Formula

   'Extract the range for the data from xVals.
   xVals = Mid(xVals, InStr(InStr(xVals, ","), xVals, _
      Mid(Left(xVals, InStr(xVals, "!") - 1), 9)))
   xVals = Left(xVals, InStr(InStr(xVals, "!"), xVals, ",") - 1)
   Do While Left(xVals, 1) = ","
      xVals = Mid(xVals, 2)
   Loop

   'Attach a label to each data point in the chart.
   For Counter = 1 To Range(xVals).Cells.Count
     ActiveChart.SeriesCollection(1).Points(Counter).HasDataLabel = _
         True
      ActiveChart.SeriesCollection(1).Points(Counter).DataLabel.Text = _
         Range(xVals).Cells(Counter, 1).Offset(0, -1).Value
   Next Counter

End Sub