SeriesCollection error

[Colvic]

Hi,
Can anyone tell me why this code is not working.
I try to sett the Weight property on lines in a Excel Graph from Access. The Chart have up to 20 lines. It works until lines no 6, then I get a error message: Unable to set the Weight property of the Border class

Here is the code:

Counter = 2
While Counter < objExcelApp.ActiveChart.SeriesCollection.Count
With objExcelApp.ActiveChart.SeriesCollection(Counter).Border
.Weight = xlThick
End With
Counter = Counter + 1
Wend

Thanks in advance