Three states in DataGridViewCheckBoxColumn

I am trying to add a DataGridViewCheckBoxColumn with three states, and for some reason, all the checkboxes are in intermediate state - even if they should be in true or false state:

DataGridViewCheckBoxColumn with three states

My code is:

            List<PointTable> listOffsetSitesForDataGridView = dicOffsetSites.Values.ToList();
            DataGridViewOffsets.DataSource = listOffsetSitesForDataGridView;

            DataGridViewCheckBoxColumn chk = new DataGridViewCheckBoxColumn();
            DataGridViewOffsets.Columns.Add(chk);
            chk.ThreeState = true;
            chk.HeaderText = "Exists";
            chk.Name = "chk";
            chk.ValueType = typeof(OffsetSitesState);
            chk.TrueValue = OffsetSitesState.eAll;
            chk.FalseValue = OffsetSitesState.eNone;
            chk.IndeterminateValue = OffsetSitesState.ePartial;


            int iPointNumber = -1;
            for (int iRowNumber = 0; iRowNumber < DataGridViewOffsets.Rows.Count; iRowNumber++ )
            {
                bool bConverted = int.TryParse(DataGridViewOffsets.Rows[iRowNumber].Cells["PointNumber"].Value.ToString(), out iPointNumber);
                if (bConverted == true)
                { 
                    if (iMaxCount == dicOffsetSitesCounter[iPointNumber])
                    {
                        DataGridViewOffsets.Rows[iRowNumber].Cells["chk"].Value = OffsetSitesState.eAll;
                    }
                    else if (dicOffsetSitesCounter[iPointNumber] == 0)
                    {
                        DataGridViewOffsets.Rows[iRowNumber].Cells["chk"].Value = OffsetSitesState.eNone;
                    }
                    else
                    {
                        DataGridViewOffsets.Rows[iRowNumber].Cells["chk"].Value = OffsetSitesState.ePartial;
                    }
                }      
            }

Thanks for your help, Reut.