Django template length filter broken for _set?

Question

I use |length filter in my templates to display the recordcount of my objects, however I'm currently forced to use .count since it seems that using the filter against a related set doesn't work! The following

{{ myobject.retatedObject_set.all|length }}

prints literally:

{{ myobject.retatedObject_set.all|length }}

And this:

{{ myobject.retatedObject_set.all.count }}

returns the expected result...

BUT: count() generates an extra SQL query, which is why I always use |length filter which doesn't! (see my answer: https://stackoverflow.com/a/18578147/267719 to question django - show the length of a queryset in a view)

Can this be considered a bug in the Django template engine? How can I avoid the extra query?

EDIT:

After an hard debug I realized that the problem was the use of the django-debug-toolbar (which prints in its panels each filter used as normal strings) plus the use of AngularJS (which has the same syntax for its templates). I solved by reconfiguring AngularJS to use different symbols:

config(function($interpolateProvider) {
    $interpolateProvider.startSymbol('{$');
    $interpolateProvider.endSymbol('$}');
});