django: Invalid filter

First off remove the semicolon after your replace.

Do you have a file called __init__.py (this is suppose to have 2 underscores before and after init, hard to format in editor.) under the templatetags directory?

Here is a good page with lots of info if you haven't looked yet.

http://docs.djangoproject.com/en/dev/howto/custom-template-tags/

Just for reference, I solved the problem by moving

{% load ... %}

from the base template to the concrete template. See also this post https://stackoverflow.com/a/10427321/3198502