Is it mandatory to provide a default locale.properties or @DefaultStringValue while using gwt internationalization?

Question

I am using Static String Internationalization.

Locale.java

public interface Locale extends Constants{

String kbps();

}

Locale_en.properties

kbps=Kbps1

Both are in the same package. In .gwt.xml, included following line:

<extend-property name="locale" values="en"/>

Now when I am compiling the code, i'm getting following error:

com.google.gwt.i18n.rebind.AbstractResource$MissingResourceException: No resource found for key 'kbps'

The above error goes away if I provide Locale.properties or @DefaultStringValue for the key. any idea why the gwt compile-time error is coming even if I am specifying the locale in the module XML?