Why do I get interrogation marks around jstl fmt tag?

I have an issue. Wherever I pasted an fmt tag for example like this <fmt:message key='summary'/> and run my app, this is how it is displayed in the browser: ???summary???.

I have been looking for a solution for hours. I might be looking at the wrong spots... Any idea how to fix this?

This is the library I use:

<%@taglib prefix="fmt" uri="http://ift.tt/1bRkrVx"%>