Internal 500 error on pages with specific template

On several project overview pages I get the following error:

ActionView::Template::Error (undefined method `sha' for nil:NilClass):
7: = link_to namespace_project_edit_blob_path(@project.namespace, @project, tree_join(@repository.root_ref, readme.name)), class: 'light' do
8: %i.fa.fa-pencil
9: .wiki
10: = cache(readme_cache_key) do
11: = render_readme(readme)
12: - else
13: %h3.page-title
app/helpers/projects_helper.rb:277:in `readme_cache_key'
app/views/projects/_readme.html.haml:10:in `_app_views_projects__readme_html_haml__156802837152618802_70285362889100'
app/views/projects/show.html.haml:47:in `_app_views_projects_show_html_haml___1126358375033705154_70285364446680'
app/controllers/projects_controller.rb:85:in `block (2 levels) in show'
app/controllers/projects_controller.rb:79:in `show'

This is version 7-13-stable (6cb800a0).

A similar error is #2084 (closed) which was posted 2 days ago.