The following code is failing on the last line. Has anyone any ideas?

Dim TargetSheet As Worksheet

Set TargetSheet = Application.Workbooks("Summary.xls").Worksheets
("Sheet3")
'Add column headings
TargetSheet.Range(Cells(1, 1)).Value = "Disease Group"

In other posts I have seen people were helped out by qualifying the
cells method with the range, but when I substitute the last line with:
TargetSheet.Range(TargetSheet.Cells(1, 1)).Value = "Disease Group"
it still fails on that line with the same error.

Kind regards,
John