diff --git a/docs/requirements.txt b/docs/requirements.txt
index fd70c42..11c9bf4 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1,3 +1,5 @@
+# -*- makefile -*-
+
 Sphinx~=2.4.4
 actdiag~=2.0.0
 blockdiag~=1.5.4
@@ -18,3 +20,10 @@
 # funcparserlib=~0.3.5
 # ----------------------------------------------------------------
 funcparserlib~=1.0.0a0
+
+# ----------------------------------------------------------------
+# ImportError: cannot import name 'environmentfilter' from 'jinja2
+# ----------------------------------------------------------------
+Jinja2<3.1
+
+# [EOF]