Excel 97

All of a sudden I am getting the above error on an existing workbook
when activating and deactivating worksheets using VBA e.g.

Private Sub CommandButton3_Click()
Worksheets("WABC").Visible = True
Worksheets("WABC").Select
End Sub

The above procedure has worked previously and I cannot understand why
this is happening.

Any help would be appreciated.

Thanks
Shal