easy thumbnail, without thumbnail image works but with thumbnail, nothing shows up

Question

my image is displayed without thumbnail functionality, but as soon as I put thumbnail I get nothing.

I'm using easy thumbnail, I followed the documentation mainly. In template

{% load thumbnail %}
<img src="{{post.image|thumbnail_url:'avatar'}}" alt="" />

In setting

THUMBNAIL_ALIASES = {
    '': {
        'avatar': {'size': (50, 50), 'crop': True},
    },
    }