Why getting invalid use of null

I have the following code in the not in list event of a combobox on a form.  But when I enter a value that is not in the list I get an invalid use of null message.  ????

Private Sub cboInsurance_NotInList(NewData As String, Response As Integer)

Dim db As DAO.Database
Dim Rs As DAO.Recordset
Dim Msg As String
Dim NewID As String

On Error GoTo Err_CustomerID_NotInList

    If NewData = "" Then Exit Sub

    Msg = "'" & NewData & "' is not in the list." & vbCr & vbCr
    Msg = Msg & "Do you want to add it?"
    If MsgBox(Msg, vbQuestion + vbYesNo) = vbNo Then
        Response = acDataErrContinue
        MsgBox "Please try again."
    Else
        Set db = CurrentDb
        Set Rs = db.OpenRecordset("tblInsuranceCompaniesLU", dbOpenDynaset)

        Msg = "Please enter an insurance company name" & vbCr & "InsuranceCompany."
        NewID = Me.cboInsurance   '.Value        'InputBox(Msg)
        Rs.FindFirst BuildCriteria("InsuranceCompany", dbText, NewID)

        Do Until Rs.NoMatch
           NewID = InputBox("InsuranceCompany " & NewID & " already exists." & _
                    vbCr & vbCr & Msg, NewID & " Already Exists")
           Rs.FindFirst BuildCriteria("InsuranceCompany", dbText, NewID)
        Loop

        Rs.AddNew

        Rs![InsuranceCompany] = NewID

        Rs![InsuranceCompany] = NewData

        Rs.Update

        Response = acDataErrAdded

    End If

Exit_CustomerID_NotInList:
       Exit Sub
Err_CustomerID_NotInList:
       ' An unexpected error occurred, display the normal error message.
       MsgBox Err.Description
       ' Set the Response argument to suppress an error message and undo
       ' changes.
       Response = acDataErrContinue

End Sub

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