What status code to return if writing null value to variable is unsupported|OPC UA Standard|Forum|OPC Foundation

In the description of the AllowNulls property node (Part 3 v1.04, chapter 5.6.2 Variable NodeClass) it says “If it is set to false, the Server shall never return a null value and shall reject any request writing a null value.”. But what status code should be returned? 

We currently return BadOutOfRange when receiving a request to write a null value to ByteString variable nodes but when the CTT tries to write a null value to a ByteString variable it expects either Good or BadWriteNotSupported (test case “Attribute Services – Attribute Write Values – 006”). Is BadWriteNotSupported the right status code to return if the variable accepts writing the value in general, but does not accept null values to be written?

Thanks, Jonas