为什么DataGrid分页，删除到 mydatagrid.pagecount mod mydatagrid.pagesize =1就失败

为什么DataGrid分页，并且 可以删除记录。
删除到 mydatagrid.pagecount mod mydatagrid.pagesize =1就失败
Invalid CurrentPageIndex value. It must be >= 0 and < the PageCount

举个例子 ，数据库有9条记录，4条分一页，当删除第三页（第9条记录）
即出现以上错误！
但如果在第二页或第一页删除就没有出错信息！
具体怎么改？

'------------------------绑定Mydgrd 数据（既绑定用户的留言） ----------
'----------------------------------------------------------------->>>
Sub Bind_Mydgrd()
MyConnection = New SqlConnection(dbconn)
Myada =New SqlDataAdapter("select * from electro_gov",MyConnection)
Myds = New DataSet
Myada.Fill(Myds,"info")
Mydgrd.DataSource = Myds.Tables("info").DefaultView
Mydgrd.DataBind()
MyConnection.Close()
ShowCounts()
End Sub
'<<<--------------------------------------------------------------------
'----------------------------------------------------------------------

'-------------------------------------------------------------------------
'------------------------删除Mydgrd 数据（既删除用户的留言）-------------->>>
Sub delete_it(S As Object,E As EventArgs)
warn.Text =nothing
Strsql ="delete from electro_gov where id= " & s.CommandArgument
MyConnection = New SqlConnection(dbconn)
Mycmd =New SqlCommand(Strsql ,MyConnection)
MyConnection.Open()
Try
Mycmd.ExecuteNonQuery()
Catch Exc As SQLException
warn.Text ="数据库操作（删除）失败!"
warn.ForeColor=System.Drawing.Color.Red
Finally
MyConnection.Close()
End Try
'Mydgrd.CurrentPageIndex=0
Bind_Mydgrd()
End Sub
'<<<------------------------------------------------------------------------------
'------------------------------------------------------------------------------
Sub Mydgrd_Page(sender As Object, e As DataGridPageChangedEventArgs)
Mydgrd.CurrentPageIndex = e.NewPageIndex
Bind_Mydgrd()
End Sub

<asp:label ID="warn" Font-Size="9" runat="server"/><asp:label ID="page_count" Font-Size="9" runat="server"/>
<asp:datagrid
ID="Mydgrd"
AutoGenerateColumns="false"
BorderWidth="0"
Width="100%"
ItemStyle-VerticalAlign="top"
ShowHeader="false"
DataKeyField="id"
AllowPaging="True"

PageSize="4"
PagerStyle-Mode="NumericPages"
OnPageIndexChanged="Mydgrd_Page"
runat="server">
<columns>
<asp:templatecolumn>
<itemtemplate>
</datagrid>