log pattern - servlet context name with fallback on pid

Hello,

I'm trying to make a log pattern showing the servlet context name or, if
missing, showing the process id.

${web:servletContextName} %msg
works well for servlet context name alone.

%pid %msg
works well for process id alone.

${web:servletContextName:-${sys:user.name}} %msg
works well for servlet context name with fallback to username system
property ,

${web:servletContextName:-%pid} %msg
does work for servlet context name with fallback to process id.

Am I missing something in my pattern?

Thank you