dbnull in a combobox

[EyeTalion]

I'm using the code below to fill in a combox. The field can be NULL and I get an error msg when it is. I'm using the code below, but am having a problem on the line where I'm adding string.empty
(cmbAccount.Items.Add = String.Empty)...do I have this coded right?

Do While objDR.Read = True
                    If (objDR("vendor_name")) Is DBNull.Value Then
                        cmbAccount.Items.Add = String.Empty
                    Else
                        Me.cmbAccount.Items.Add(RTrim(objDR("vendor_name")))
                    End If