Grid Selected RowIndex -1 after some row

Hi,

I have a problem on using Checked Grid.
Here is my grid and a button for counting checked data.

HTML Code:

---

    <ext:GridPanel
        runat="server"
        ID="gridTest"
        ClientIDMode="Static">
        <Store>
            <ext:Store
                runat="server"
                ID="storeTest">
                <Model>
                    <ext:Model runat="server">
                        <Fields>
                            <ext:ModelField Name="name" />
                        </Fields>
                    </ext:Model>
                </Model>
            </ext:Store>
        </Store>
        <ColumnModel>
            <Columns>
                <ext:Column runat="server" Text="Name" DataIndex="name" Flex="1" />
            </Columns>
        </ColumnModel>
        <SelectionModel>
            <ext:CheckboxSelectionModel runat="server" Mode="Multi" />
        </SelectionModel>
        <TopBar>
            <ext:Toolbar
                runat="server">
                <Items>
                    <ext:Button
                        runat="server"
                        ID="BtnCheck"
                        Text="Check">
                        <DirectEvents>
                            <Click OnEvent="BtnCheck_Click" />
                        </DirectEvents>
                    </ext:Button>
                </Items>
            </ext:Toolbar>
        </TopBar>
    </ext:GridPanel>

---

I fill the grid with 100 data with this code in Page Load event.

Code:

---

        If Not Ext.Net.X.IsAjaxRequest Then
            Dim testData As New List(Of Object())

            For i As Integer = 1 To 100
                testData.Add(New Object() {"Test Data " & i})
            Next

            storeTest.DataSource = testData.ToArray()
            storeTest.DataBind()
        End If

---

As I mention before, I have a button to counting Checked data.
Here is my code to count the checked data.

Code:

---

    Protected Sub BtnCheck_Click(sender As Object, e As DirectEventArgs)
        Dim sm As RowSelectionModel = gridTest.GetSelectionModel()

        Dim counter As Integer = 0
        Dim counterAll As Integer = 0
        For Each selected As SelectedRow In sm.SelectedRows
            If selected.RowIndex >= 0 Then
                counter = counter + 1
            End If

            counterAll = counterAll + 1
        Next

        Ext.Net.X.Msg.Alert("Total Selected", counter & " Check Counted, " & counterAll & " All Checked").Show()
    End Sub

---

The problem is, when checked all, even though the data is 100, counterAll is equal to 100 but counter is equal to 53 (maybe different). The Rest of the selected data is not counted because selected.RowIndex equal to -1.
But this only happen when check all using CTRL+A or check checkbox beside header column. When checking one by one, it works perfectly.

Attachment 24837

Any idea why this is happen?