diff --git a/docs/requirements.txt b/docs/requirements.txt
index f7d101b..ebfb689 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -4,7 +4,7 @@ alabaster
 ipykernel
 git+https://github.com/spatialaudio/nbsphinx.git@master
 numpydoc
-matplotlib=2.2.3
+matplotlib==2.2.3
 scipy>=1.1.0
 numpy>=1.15
 ecos