Blame results in 500 error (on files with specific extension?)

When running Blame on a file in one of my repositories, I'm getting a 500 error. It's not caused by the fact that blame doesn't work, because another file in the same repository does show good blame output.

Possibly, this is due to the .lisp extension, because in another repository, we have a lisp file without the .lisp extension which doesn't result in the same problem.

This is what I'm seeing in the 'production.log':

Started GET "/asdf/asdf/blame/master/find-component.lisp" for 94.209.179.229 at 2015-04-27 14:08:47 +0000
Processing by Projects::BlameController#show as HTML
Parameters: {"namespace_id"=>"asdf", "project_id"=>"asdf", "id"=>"master/find-component.lisp"}
Completed 500 Internal Server Error in 160ms
ActionView::Template::Error (empty stack!):
app/helpers/blob_helper.rb:16:in `highlight'
app/views/projects/blame/show.html.haml:39:in `block (2 levels) in _app_views_projects_blame_show_html_haml___3986940253670482042_34533560'
app/views/projects/blame/show.html.haml:38:in `each'
app/views/projects/blame/show.html.haml:38:in `block in _app_views_projects_blame_show_html_haml___3986940253670482042_34533560'
app/views/projects/blame/show.html.haml:14:in `_app_views_projects_blame_show_html_haml___3986940253670482042_34533560'

Note, I'm using omnibus 7.10.0