Internal error including a EJS template in Express 2.5

I have a node.js app that uses EJS as template engine and ExpressJS 2.5.8

I try to include a partial inside a ejs page in this way:

<%- partial('list') %>

I get a "500 Internal Error" . I've tried in many different ways , also using <% include list %> but i always get a 500 error. What am i doing wrong?