• Parameterize a blog posts service (there is one in the welcome page of a blog, or you can add it in another page)
• In the type parameter, select 'Posts of a month'
• Let the year and month parameters empty
• Valid the form => there is no error at the form validation (KO)
• There are no posts displayed in the list (OK)

I think the year and month parameters should be mandatory with disable conditions on the type parameter