'markup_tags' is not a valid tag library

Everytime I add a particular object to my database (this only affects one particular app on my site), I get the TemplateSyntaxError 'markup_tags' is not a valid tag library.

Specifically, it cannot load this: {% load markup_tags %}. But this is strange because when I open a Django shell and import markup all is fine.

This question has been asked before on StackOverflow here: Django markup templatetags error

However, I don't understand why this is happening and I don't really understand how to fix it. They said render_to_response the template. But I'm not sure exactly how I would do this from the shell (Django noob here).

So the problem was that I was actually not properly importing the module into into my project that contained markup_tags. While the library was available in my environment, in my settings.py I did not properly reference it. Dumb mistake, but might help someone else.