Unknown function: _default

Just wondering if anyone can help me, I am getting the error Unkown function: _default in the following code on the line that is highlighted.

Sub UpdateStock()
'
Dim c As Range
Dim QTYSold As Integer
Dim count As Integer

QTYSold = 0

For i = 1 To 100

    If Sheets("Sales").Cells(i, "B") = "RUS" Then

      QTYSold = QTYSold + (Sheet2.Cells(i, "C"))
      Sheet1.Cells(2, "D") = (Sheet1.Cells(2, "C") - QTYSold)

    End If

Next i

End Sub