diff --git a/docs/requirements.txt b/docs/requirements.txt
index ef36add..38d2474 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -1 +1 @@
-sphinxcontrib-bibtex
+sphinxcontrib-bibtex==2.6.3
diff --git a/requirements.txt b/requirements.txt
index def87af..9e688a9 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,4 @@
 # Library dependencies for the Python code.  You need to install these with
 # `pip install -r requirements.txt` before you can run this.
 
-six
+six==1.16.0