diff --git a/requirements.txt b/requirements.txt
index 274d6578d..04ff1f671 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -22,4 +22,4 @@
 mkdocs>=1.0
 Pygments>=2.4
 markdown>=3.2
-pymdown-extensions>=6.3
+pymdown-extensions>=6.3,<7