When using a datagridview, which contains one DataGridViewComboBoxColumn, I
get an error (System.FormatException: DataGridViewComboBoxCell value is not
valid), which has something to do with the databinding.

Situation:
I create a tabledef:
Dim dt As DataTable
dt = New DataTable(TABELDISPLAY)
dt.Columns.Add("CostplanID", GetType(Long))
dt.Columns.Add("CosttypeID", GetType(Long))
gDatasetDisplay.Tables.Add(dt)

I fill it:
 For Each MyRow As DataRow In gdbProject.Costplan.GetRows()
   dr = gDatasetDisplay.Tables(TABELDISPLAY).NewRow()
   dr("CostplanID") =  gdbProject.Costplan.CostplanID(MyRow)
   dr("CosttypeID") =  gdbProject.Costplan.GetCosttypeID(CostplanID)
   gDatasetDisplay.Tables(TABELDISPLAY).Rows.Add(dr)
 Next
 MyDataView = New DataView(gDatasetDisplay.Tables(TABELDISPLAY), "", "",
DataViewRowState.CurrentRows) '"[MilestoneID]=" & MilestoneID & " AND
[PartnerID]=" & PartnerID
 MyDataGridView.DataSource = MyDataView

I add a combo to the list, which should be linked to CosttypeID
 Dim List As New DataGridViewComboBoxColumn()
 List.HeaderText = "Costtype"
 List.DataPropertyName = "CosttypeID"
 DBProgram.Costtype.GetCosttypes(List)
 .Columns.Add(List)

This however does not seem to work. What am I doing wrong???

The odd thing however, If I use the following code (which I do not WANT to
use), it shows the combobox without the error!:

 gDatasetDisplay = New DataSet
 Dim MyDBLink As New DBLink(DBProgram.GetConnectionString, QUERYDISPLAY,  
TABELDISPLAY)
 MyDBLink.OpenRecordset(gDatasetDisplay)
 MyDBLink.Dispose()
 MyDataView = New DataView(gDatasetDisplay.Tables(TABELDISPLAY), "", "",
DataViewRowState.CurrentRows) '"[MilestoneID]=" & MilestoneID & " AND
[PartnerID]=" & PartnerID
 MyDataGridView.DataSource = MyDataView