diff --git a/docs-requirements.in b/docs-requirements.in
--- a/docs-requirements.in
+++ b/docs-requirements.in
@@ -1,10 +1,11 @@
 sphinxcontrib-blockdiag
 sphinxcontrib-nwdiag
 sphinxcontrib-seqdiag
 Sphinx
 sphinx-rtd-theme
+derp
 
 # Generating Python module documentation requires importing these packages.
 backports.lzma
 docker<2.6
 coverage