Google Analytics Enhanced Ecommerce doesn't Track Price

Question

I'd like to track pledges with GA-EC, however, since these are not purchases, some fields have to be used in a rather creative fashion. Nevertheless, the following tracker code does work in parts:

(function(i, s, o, g, r, a, m) {
  i['GoogleAnalyticsObject'] = r;
  i[r] = i[r] || function() {
    (i[r].q = i[r].q || []).push(arguments)
  }, i[r].l = 1 * new Date();
  a = s.createElement(o),
    m = s.getElementsByTagName(o)[0];
  a.async = 1;
  a.src = g;
  m.parentNode.insertBefore(a, m)
})(window, document, 'script', '//www.google-analytics.com/analytics.js', 'ga');
ga('create', "UA-XXXXXXXX-X", 'auto');
ga('require', 'displayfeatures');
ga('require', 'ec');

   (...)

ga('ec:addProduct', {
  "id":"Reward@123",
  "name":"50",
  "category":"music",
  "currency":"EUR",
  "price":"50"
});
ga('ec:setAction', 'purchase', {
  "id":"Payment@123",
  "affiliation":"Project@123",
  "coupon":"prepaid"
});

   (...)

ga('send', 'pageview');

I do see the affiliation or the coupon, but the amount is always 0, no matter which currency is selected.

Any idea what's I'm doing wrong here?