Error If I Enter Wrong Criteria

The following code works like a dream for me, but if was to enter the wrong letter, not "N" or "R" and i tried to delete it, a debug message appears?

Is there a way to allow any letters to be deleted?

Here is my code

Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Column = 17 Then
With Range("C" & Target.Row & ":R" & Target.Row).Interior
Select Case Target.Value
Case "R": .Color = RGB(184, 204, 228)
Case "N": .Color = RGB(120, 120, 120)
Case Else: .Color = RGB(255, 255, 255)
End Select
End With
With Target.Font
Select Case Target.Value
Case "R": .Color = RGB(184, 204, 228)
Case "N": .Color = RGB(120, 120, 120)
Case Else: .Color = RGB(0, 0, 0)
End Select
End With
End If
End Sub