diff --git a/requirements.txt b/requirements.txt
index d984cc70aa265dc95807aebe7e66ba1971d9fb88..21385fc2f0ef207960f072950b23a6d7cd68ccd3 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,6 +9,7 @@ sphinxcontrib-napoleon>=0.7
 Pygments>=2.4.2
 flake8>=3.8.3
 sphinx-book-theme>=0.2.0
+beautifulsoup4<=4.12.3  # to work with python3.6, required by sphinx-book-theme
 
 # IDE related (can be skipped)
 pylint>=2.3.1