Sum Lastrow formula!!

Hi, I'm not sure where I'm going wrong with this formula? I cannot specify a worksheet as they vary. Thanks

Sub Sum_Last_Row()

Set Lastrow = Cells(Rows.Count, "C").End(xlUp).Offset(1, 0)
 
Lastrow = WorksheetFunction.Sum("C7, Lastrow -1")
 
 End Sub