JAXB: how to make JAXB NOT to unmarshal empty string to 0

I have a DTO class with a field such as:

@XmlAttribute
@NotNull
private Integer number = null;

I'm trying to unmarshal xml such as

...  number=""  ...

I need the nuber field to stay null, so that a validation exception would be thrown. Instead JAXB unmarshals it as 0. How can I make it to behave correctly ?