下标越界？ 小弟我这个遍历程序错在哪里啊 求教

Sub tt()
For i = 1 To 50
For j = 1 To 50
    Workbooks("book").Worksheets("Sheet1").Cells(i, j).Value = "a"
Next
Next

End Sub

'在工程中引用microsoft office 11.0 control
Sub tt()
Dim xobject As New Excel.Application
Dim xBook As New Excel.Workbook
Dim xSheet As New Excel.Worksheet

Dim rownum as integer
Dim colnum as integer

Set xobject = CreateObject("excel.application")
Set xBook = xobject.Workbooks.Open("D:\TEST.xls")
Set xSheet = xobject.Worksheets("sheet1")
xSheet.Activate

For i = 1 To 50
    For j = 1 To 50
        xSheet。Cells(i, j).Value = "a"
   Next
Next

xBook.Save '保存

'退出excel,释放资源
Set xSheet = Nothing
xBook.Close
Set xBook = Nothing
xobject.Quit
End Sub

------解决方案--------------------
确认工作簿的名字叫book? 不是book.xls?,确认book.xls中有一张叫sheet1的工作表。

------解决方案--------------------
Workbooks("book.xls").Worksheets("Sheet1").Cells(i, j).Value = "a"