Populate DataGridVieComboBox with adaptable

[Benniit]

Please I have DatagridView bound to a DataTable
that pulls records and displays the DataGridView
correctly. Now, I have ColumnType as
DataGridViewComboBoxColumn of the forth
column named "Username". When I click the
combo box of the DataGridview, nothing gets
displayed at all. I just don't understand it.
Below is the code.

'Objects already initialized
Dim QueryU As String = "select * from Users
where username='" & frmUsers.txtUsername.Text
& "'"
Dim daUser As New SqlDataAdapter
(QueryU, SQLCon)
Dim dtUser As New DataTable
daUser.Fill(dtUser)
If dtUser.Rows.Count > 0 Then
Dim c4 As New DataGridViewComboBoxColumn()
c4.HeaderText = "Send To"
c4.Name = "Username"
c4.DataPropertyName =
"Username"
c4.DisplayMember =
"Username"
c4.ValueMember =
"Username"
c4.DisplayStyleForCurrentCellOnly = False
c4.DisplayStyle =
DataGridViewComboBoxDisplayStyle.Nothing
c4.FlatStyle =
FlatStyle.Popup
c4.SortMode =
DataGridViewColumnSortMode.Automatic
c4.DataSource = dtUser
c4.Width = 300
frmUsers.DataGridView1.Columns.Add(c4)