Expected string or buffer with django pipeline when trying to minify js

Question

When I try to this on my base template

{% load compressed %}

{% compressed_js 'app_name' %} <- fails here

This is my settings.py file:

PIPELINE_JS = {
    'app_name': {
        'source_filenames': (
          'static/js/bootstrap.js',
        )
    }
}

are this paths right?

I get:

Exception Value:    
expected string or buffer

any clues?