Check Box in DataGrid

Hello.
I'm working in a Windows Form (VS .Net 2003), I have included a DataGrid and one of the colum is a CheckBox, my problem is that in run time, the check box colum appears gray, like in three states, bu if Click over one of this and debug the code, I find that VarType(DG_Detalle.Item(Nro_Fil, Nro_Col)) = vbNull.

I have tried of everything, need help.

the next is the definition:
Dim Chk_DarBaja As DataGridColumnStyle = New DataGridBoolColumn
Chk_DarBaja.MappingName = "Chk_DarBaja"
Chk_DarBaja.HeaderText = "Dar Baja"
Chk_DarBaja.ReadOnly = False
Chk_DarBaja.Width = 50
DGTStyle.GridColumnStyles.Add(Chk_DarBaja)

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